Two babies born to a mother at one birth. There are two types of twins, monozygotic and dizygotic. Members of a twin pair are called co-twins.
Controversy surrounding the definition of a twin arose with the advent of reproductive technologies enabling the simultaneous fertilization of eggs, with separate implantation. The unique “twinlike” relationships that would result between parents and cloned children (who would be genetically identical to their parents) also challenge current conceptions of twinship. Monozygotic twins are clones (genetically identical individuals derived from a single fertilized egg), but parents and cloned children would not be twins for several reasons, such as their differing prenatal and postnatal environments. See alsoReproductive technology.
Monozygotic twins
The division of a single fertilized egg (or zygote) between 1 and 14 days postconception results in monozygotic twins. They share virtually all their genes and, with very rare exception due to unusual embryological events, are of the same sex. A common assumption is that because monozygotic co-twins have a shared heredity, their behavioral or physical differences are fully explained by environmental factors. However, monozygotic twins are never exactly alike in any measured trait, and may even differ for genetic reasons.
Sometimes chromosomes fail to separate after fertilization, causing some cells to contain the normal chromosome number (46) and others to contain an abnormal number. This process, mosaicism, results in monozygotic co-twins who differ in chromosomal constitution. There are several other intriguing variations of monozygotic twinning. Splitting of the zygote after day 7 or 8 may lead to mirror-image reversal in certain traits, such as handedness or direction of hair whorl. The timing of zygotic division has also been associated with placentation. Monozygotic twins resulting from earlier zygotic division have separate placentae and fetal membranes (chorion and amnion), while monozygotic twins resulting from later zygotic division share some or all of these structures. Should the zygote divide after 14 days, the twins may fail to separate completely. This process, known as conjoined twinning, occurs in approximately 1 monozygotic twin birth out of 200. The many varieties of conjoined twins differ as to the nature and extent of their shared anatomy. Approximately 70% of such twins are female. There do not appear to be any predisposing factors to conjoined twinning. See alsoMosaicism.
Dizygotic twins result when two different eggs undergo fertilization by two different spermatozoa, not necessarily at the same time. Dizygotic twins share, on average, 50% of their genes, by descent, so that the genetic relationship between dizygotic co-twins is exactly the same as that of ordinary brothers or sisters. Dizygotic twins may be of the same or opposite sex, outcomes that occur with approximately equal frequency.
There are some unusual variations of dizygotic twinning. There is the possibility of polar body twinning, whereby divisions of the ovum prior to fertilization by separate spermatozoa could result in twins whose genetic relatedness falls between that of monozygotic and dizygotic twins. Blood chimerism, another variation, refers to the presence of more than one distinct red blood cell population, derived from two zygotes, and has been explained by connections between two placentae. In humans, chimerism can occur in dizygotic twins. Superfecundation is the conception of dizygotic twins following separate fertilizations, usually within several days, in which case each co-twin could have a different father. Superfetation, which refers to multiple conceptions occurring several weeks or even one month apart, may be evidenced by delivery of full-term infants separated by weeks or months and by the birth or abortion of twin infants displaying differential developmental status. See alsoOogenesis.
Epidemiology
According to conventional twinning rates, monozygotic twins represent approximately one-third of twins born in Caucasian populations and occur at a rate of 3–4 per 1000 births. The biological events responsible for monozygotic twinning are not well understood. It is generally agreed that monozygotic twinning occurs randomly and not as a genetically transmitted tendency. Some recent evidence from Sweden suggests an increased tendency for mothers who are monozygotic twins to bear same-sex twins themselves; further work will be needed to resolve this question.
Dizygotic twinning represents approximately two-thirds of twins born in Caucasian populations. The dizygotic twinning rate is lowest among Asian populations (2 per 1000 births), intermediate among Caucasian populations (8 per 1000 births), and highest among African populations (50 per 1000 births in parts of Nigeria). The natural twinning rate increases with maternal age, up to between 35 and 39 years, and then declines. Dizygotic twinning has also been linked to increased parity, or the number of children to which a woman has previously given birth. Mothers of dizygotic twins are also significantly taller and heavier, on average, than mothers of monozygotic twins and singletons. Dizygotic twinning appears to be genetically influenced, although the pattern of transmission within families is unknown.
Twinning rates have risen dramatically since about 1980 mainly due to advances in fertility treatments (for example, in vitro fertilization and ovulation induction), but also due to delays in the child-bearing years. The increase has mainly involved dizygotic twinning in which multiple ovulation and maternal age matter.
Twins are of two types: monovular (identical), from the union of one sperm and one ovum, and binovular (non-identical) resulting from the fertilization of two separate ova. The cell produced by fertilization is called a zygote (from the Greek for ‘yoked’), so they are also known as monozygotic and dizygotic. Dizygotic twins are physically and genetically as dissimilar as any siblings. Monozygotic twins, having resulted from the cleavage of a single ‘conceptus’ — the splitting and separation of an early embryo — are therefore, with rare exceptions, genetically identical.
The incidence of multiple pregnancies varies in different racial groups. To quote ‘Hellin's law’ (1895): ‘twins occur in 1/89 births, triplets 1/ (89) 2, quadruplets 1/(89) 3 and so on’. The formula is roughly correct, although twins occur in Caucasians 1/80 to 1/90, in Asiatics 1/150 or less, and black Africans 1/50 with the highest incidence of twinning amongst the Yoruba people of Nigeria for whom 1 in 25 births are of twins. It is the rate of non-identical (dizygotic) twinning that varies around the world: identical (monozygotic) twins occur at a similar rate of 1 in 300 births in all populations. These statistics are based on clinical findings in viable pregnancies. However the initial ‘hidden’ twinning rate is probably higher: with increasing use of ultrasound in early pregnancy it is found that before 12 weeks one of the twins may die and be absorbed leaving an apparent singleton. In Australia the rate of twinning has increased approximately 25% over the past 20 years, partly due to a significant increase in the percentage of births to women aged 35 and over, and partly to the treatment of infertility by ovulation stimulation or assisted conception by gamete intra-fallopian transfer (GIFT) or in-vitro fertilization (IVF).
Twin pregnancy is more prone to complication than single pregnancies and possible hazards of premature birth and poor growth in the womb necessitate increased antenatal surveillance. If twins are identical and they share a single placenta, one baby can steal blood from the other, causing a condition known as ‘Twin- twin transfusion syndrome’.
Multiple pregnancies carry a greater risk of losing a baby before, during, or after birth than singleton pregnancies: multiple pregnancies overall account for more than 10% of all perinatal deaths; the greater the number, the greater the risk. Cerebral palsy in survivors is six times more common in twins than singletons.
The birth of twins has been a source of fascination in many cultures throughout history and the twin image has been incorporated in myths, folklore, and religions. The Old Testament of the Bible tells of Isaac's wife, Rebekah, who eventually conceived after nineteen years of marriage. Twin boys were born. The first was red and hairy, and he was named Esau, meaning ‘red’. His brother was born holding Esau's heel and so he was called Jacob, meaning ‘he who grasps the heel’. Ancient Rome was founded, according to legend by Romulus and Remus, the twin progeny of Mars, god of war, and a mortal princess. In some African communities, twins were regarded with great favour; in others, with great suspicion. The Yoruba in Nigeria were well aware of the high mortality associated with twinning in the past, and they made small wooden sculptures, ‘ibeji’ that had spiritual significance if one of twins died.
Two siblings produced in the same pregnancy and developed from one egg (identical, monozygotic) or from two eggs fertilized at the same time (fraternal, dizygotic).
Twins occur when two babies are born at the same birth.
Description
Identical, or monozygotic, twins are of the same sex and are genetically identical and physically similar, because they both come from one ovum (egg), which, after fertilization, divides in two and develops into two separate fetuses. Fraternal, or dizygotic, twins occur when the mother produces two eggs in one monthly cycle and both eggs are fertilized. The conceptions may take place on two separate occasions and could involve different fathers.
Fertilized egg division which produces twins can either happen early or late in development. In the case of early separation, the two fetuses either share an amniotic sac or each has a separate amniotic sac. If the fetuses share an amniotic sac, they also share a placenta. If the two fetuses have separate amniotic sacs, they can either share a placenta or have two separate placentas. Twins can also result from a fertilized egg that divides slightly later in development. In this case, the twins share an amniotic sac and a placenta. It is from these cases of late separation that conjoined (Siamese) twins sometimes develop.
Fraternal twins, who are no more genetically alike than ordinary siblings, may be of the same or different sex and may bear some similarity of appearance. Fraternal twinning appears to be passed on by the female members of a family. If the mother is a fraternal twin herself, has fraternal twin siblings, or fraternal twin relatives on her side of the family, she is more likely to give birth to fraternal twins. If she has already given birth to fraternal twins, her chances of giving birth to fraternal twins again are four times greater than those of a woman who has not had fraternal twins. In vitro fertilization increases a woman's chances for having multiple birth.
The number of twins born in the United States rose between the early 1980s and the early 2000s. In 1980, there were 69,339 sets of twins born, and in 2002 there were 125,134 sets of twins born in the United States. According to data gathered by the Centers for Disease Control (CDC), there is considerable variation among the states in number and rate of twin births. In 1994, for example, the twin birth rate ranged from 19.8 per 1,000 live births in Idaho and New Mexico to 27.7 per thousand in Connecticut and Massachusetts. One factor that may influence the distribution of multiple births is whether the state provides insurance coverage for procedures such as in vitro fertilization (IVF) and other treatments to improve fertility. These procedures increase the chance of multiple births.
Ethnicity is another factor that may correlate to the twin birth rate. For 1994, the twin birth rate among non-Hispanic white mothers was 24.3 per 1000 live births; among non-Hispanic black mothers, 28.3 per 1000; and among Hispanic mothers, 18.6 per 1000. There are also significant differences internationally in the number of twins born with the rate in Belgium almost six times the rate in China.
The CDC also studies whether maternal age has any correlation with the rate of twin births. The data seem to suggest that mothers in states with rates of twin births higher than the overall rate for the United States are older on average, and mothers in states with rates of twin births lower than the overall rate for the United States are younger. Again, as in vitro fertilization is more widely done, the incidence of multiple births will increase.
Infancy
Parents should avoid giving twins very similar names. Twins should be treated as two individuals and not as a package. They may need to be fed at different times and may develop skills at different rates. It is important to spend time with each twin separately so that they become used to being separated from each other for short times and know that they are each valued as individuals.
Toddlerhood
To help twins understand who they are as individuals, parents should avoid dressing both twins the same. It is preferable that each child receive toys that are geared towards their individual interests rather than each receiving the same toy.
School Age
Sibling rivalry can be more intense in twins than in siblings of different ages. This is not unusual, because teachers, coaches, and even parents tend to compare twins. All children compare themselves to their siblings, and having others do this regularly can add to the pressure and stress of being a twin. Parents should consider arranging to have the twins put in different classes in school to help foster individuality. Each twin will probably have different skills, interests, and friends, and they should be encouraged to peruse activities separately if their interests diverge. Helping teachers, coaches, babysitters, and friends understand that it is important to treat the twins as two separate people can be very important. Friends should be encouraged to give separate gifts for birthdays and holidays, taking each child's special interests and talents into account.
Common Problems
Twins often have a harder time developing their own independent identities than other children. Twins are more likely to have low birth weights or be delivered prematurely than single babies.
Parental Concerns
Raising twins can be more challenging than raising two single children. The children may need to eat, sleep, and be changed at different times when they are infants. It can also be more expensive, because things like car seats and cribs must be purchased at the same time instead of reused for the second child. Some stores have special discounts for parents of twins.
When to Call the Doctor
Parents should call the doctor if one or both of their children seems ill, just as they would for any other child or children.
Resources
Books
Noble, Elizabeth, with Leo Sorger. Having Twins and More: A Parent's Guide to Multiple Pregnancy, Birth, and Early Childhood, 3 ed. Boston, MA: Houghton Mifflin, 2003.
Pearlman, Eileen M., and Jill A. Ganon. Raising Twins: What Parents Want to Know, and What Twins Want to Tell Them. New York: Harper Resource, 2000.
Periodicals
Brown, Judith E., and Marcia Carlson. "Nutrition and Multifetal Pregnancy." Journal of the American Dietetic Association 100 (March 2000): 343.
Organizations
National Organization of Mothers of Twins Clubs. PO Box 438, Thompsons Station, TN 37179–0438. Web site: www.nomotc.org.
To estimate the relative contributions of genes and environment to the cause of disease by comparing MZ to DZ concordance;
To investigate environmental determinants of etiology in discordant twin pair studies;
To investigate environmental influences on disease course in concordant twin pair studies;
To characterize "presymptomatic" or "at risk" states by studying the unaffected twins in discordant pairs.
Twins are siblings carried together in the womb and born at the same time. Similarities and differences between twins can be used to answer questions about the role genes and the environment play in the development of traits such as personality, intelligence, and susceptibility to disease. While results from any single pair of twins cannot provide conclusive answers to such questions, the study of large numbers of twin pairs allows researchers to draw conclusions about inheritance with a significant degree of confidence.
Developmental Mechanisms
Twins are classified as either dizygotic or monozygotic. Dizygotic twins (also called fraternal twins) arise from two separately fertilized eggs, or zygotes. In humans, usually only one egg is released at a time from a woman's ovaries. When two are released, both may become fertilized by separate sperm and implant in the uterus. Dizygotic twins develop separate placentas and amniotic sacs. They may be of the same or different sexes. In the absence of reproductive technology interventions, dizygotic twinning occurs in approximately three of every thousand human births, a rate that increases with maternal age, varies with ethnic group, and is probably influenced by genes that control pituitary function. Various types of assisted reproductive technologies routinely create dizygotic twins, triplets, and higher numbers of offspring.
Monozygotic twins (also called identical twins) arise from a single fertilized egg. At some point after the zygote begins to divide, the cell mass splits into two, creating two embryos from one. Monozygotic twinning occurs in approximately 0.25 percent of human births. Monozygotic twins are always of the same sex. If the cell mass splits before about day five after fertilization, the two embryos will develop with separate placentas and separate amniotic sacs. This occurs in about two-thirds of human monozygotic twins. Between day five and about day nine, splitting leads to two amniotic sacs but one placenta. This occurs in about one-third of Monozygotic twins. Twins that split after day nine will share the amniotic sac. Splitting that late also increases the likelihood that the twins will not separate completely and will develop into conjoined (Siamese) twins.
Monozygotic Versus Dizygotic Twins
Because monozygotic (MZ) twins develop from a single fertilized egg, they begin life with exactly the same set of genes. In this respect, they are clones—organisms whose genes are identical. As discussed below, however, they may accumulate genetic and other differences during development.
In contrast, dizygotic (DZ) twins are no more genetically close than any pair of siblings. While it is commonly said that siblings share half their genes, this is incorrect for two reasons. First, the random nature of meiosis and fertilization means that two siblings could end up with many, or few, genes from a particular parent in common. Second, there are many human genes for which there is only one common form, or allele. Therefore, any two people will share many alleles, regardless of their relationship. Only those genes with more than one allele form the basis of human genetic variation. These are the real focus of the question about gene-sharing in siblings. Of these variable genes, siblings (including dizygotic twins) on average share half.
Because dizygotic twins are the same age, they may share more of their environment than would two siblings of different ages. For instance, because they are likely to be engaged in similar activities, dizygotic twins are more likely to have similar environmental exposures (including behaviors, diet, hobbies, exposure to infectious agents, and exposure to chemicals)—whether at home, at school, or in the community—than two siblings of different ages and different activity patterns. It is this similarity of environment but difference of genes that makes them a useful contrast to monozygotic twins, whose environments and genes are largely identical.
Similarities and Differences Between Monozygotic Twins
The fertilized egg cell that gives rise to MZ twins begins life with a single set of genes, and so we might predict that every cell that arises from it would be exactly identical. However, small differences between daughter cells may accumulate throughout embryonic development and later in life. The earliest difference may be in the mitochondria each inherits. Mitochondria are the cell's power plants and contain a small amount of DNA. Some of the hundreds of mitochondria in a cell may contain mutations. If the cells that create the two twins carry different mitochondrial genes, even identical twins will be genetically different. Mutations can also accumulate during embryonic development, or after birth, either in the mitochondrial genes or the genes in the nucleus. Such mutations may have a significant effect: Some types of cancer are due to mutations accumulated during one's lifetime, often through exposure to environmental chemicals or radiation.
For the vast majority of genes, though, MZ twins are exactly identical. Nonetheless, twins do experience slightly different environments, even when reared together, and any early differences between them may be accentuated by families members, or by one another, leading to development of very different personalities.
Amazing Twin Similarities
Some of the most tantalizing clues to the genetic basis of human personality and behavior come from studies of MZ twins reared apart since birth. Such twins have the same genes but, presumably, different environments. A major study of more than 100 such twin pairs showed some remarkable coincidences. A pair of twins meeting for the first time at age thirty-nine each arrived wearing seven rings, two bracelets on one wrist, and a watch and one bracelet on the other wrist. Another twin pair discovered they each had dogs named Toy, had married and divorced women named Linda, remarried women named Betty, and named their sons James Allan and James Alan.
Table 1
CONCORDANCE IN TWIN STUDIES
Pairwise concordance
Number of twin pairs in which both are affected Total number of twin pairs
Proband-wise Concordance
[2c2+ c1] [2c2+ c1+ d]
A proband is an independently ascertained twin with the disease; independently ascertained means the twin was NOT identified through the co-twin.
c2 = the number of concordant pairs in which both twins are probands
c1 = the number of concordant pairs in which only one twin is a proband
d = the number of discordant pairs
Using concordance patterns to estimate the relative contributions of genetic and environmental determinants to a condition or disorder:
If MZ concordance = 100%
Only genetic determinants likely
If MZ > DZ concordance
Genetic determinants important
Environmental modifiers likely
If MZ concordance = DZ concordance
Shared environmental determinants likely
While these coincidences are amazing, it is important to remember that many more twin pairs in this study did not have such parallel lives or habits. Such stories are curious and provocative but cannot by themselves tell us about the relative contributions of genetics and the environment in shaping personality, behavior, health, or other aspects of the self.
Twin Studies and Concordance
Insight into such questions can be gleaned by several types of studies that compare twins. Comparison of MZ twins reared apart is one type of study but is hampered by the extreme rarity of such twin pairs. Another type of study, comparing MZ twins to DZ twins, is more commonly done, because there are many hundreds of thousands of such twin pairs worldwide. Data on twins have been collected by numerous research groups who have created large and growing databases (registries) that can be mined for information.
Determining a characteristic called concordance plays a crucial role in most such studies. A twin pair is said to be concordant for a trait if both members show it. If neither twin shows the trait, the pair is also concordant, but for the absence of the trait. For instance, twins are concordant for Alzheimer's disease if both develop it. They are discordant if one does have the disease but the other does not.
Table 2
PAIRWISE CONCORDANCE FOR PARKINSON'S DISEASE
Concordant Pairs
Discordant Pairs
Pairwise Concordance
Risk of Concordance if MZ
MZ
DZ
MZ
DZ
MZ
DZ
RR (95% CI)
Overall/span>
11
10
60
80
15.5%
11.1%
1.39(0.63-3.10)
First twindiagnosed <50
4
2
0
10
100.0%
16.7%
6.00 (1.69-21.3)
First twin diagnosed >50
7
8
58
68
10.8%
10.5%
1.02 (0.39-2.67)
If a trait is strongly influenced by genes, more MZ twin pairs should be concordant than DZ twin pairs, because MZ twins share more genes. Comparing concordance rates between the two groups, and applying some mathematical analysis, allows researchers to estimate the genetic contribution to a trait, as shown in Table 1.
Twin Registries
Twin studies can have several starting points. Some investigators begin simply by trying to identify twins who will volunteer to be part of a particular research study. Often twins are sought by advertising for twins with the particular disease of interest. This approach has the advantage of simplicity, as twins identify themselves to the research team.
However, twins who volunteer may differ in some important way from those who do not volunteer, and this could affect the conclusions drawn from the study. For example, MZ twins are more likely to volunteer, in general, than DZ twins are. This tendency to volunteer for twin studies among MZ twins is probably because being a twin is a more central part of the identity of MZ pairs than DZ pairs. Also, twins concordant for a particular disease are more likely to volunteer than those without the disease are. If both influences are at work in the same study, more concordant MZ twins than DZ twins may be identified, not because there is an actual difference in concordance between MZ and DZ twins (and thus a genetic effect at work), but because more concordant MZ twins volunteered for the study. If this pattern of volunteerism is mistaken to represent the true pattern of the disease in all twins, an inappropriate conclusion that the disease has genetic causes could result.
Other twin registries attempt to identify all twins within a particular population. One approach is the statewide or national twin registry. All twin births in the region are reported to a central registrar. This results in a more complete picture of all twin pairs in these populations. Examples include the statewide Virginia and Minnesota twin registries in the United States and many national twin registries, including those in the United Kingdom, Australia, the Scandinavian countries, Germany, Belgium, the Netherlands, Italy, and Sri Lanka.
Twin registries have also been assembled from among special populations. Examples in the United States are registries assembled from military records (the World War II Veteran Twins Registry and the Vietnam Era Twin Registry) and from Medicare files (the U.S. Registry of Elderly African-American Twins). In these registries, likely adult twins were identified by searching records to identify individuals with identical dates of birth, birthplaces, and surnames. These individuals were then contacted to verify whether they actually constituted a twin pair. Registries may also be established by identifying twin births within a health maintenance organization (such as the Kaiser Permanente Twin Cohort, in California).
Each registry varies in the amount of contact with registrants. In all, individual contact is strictly monitored to preserve the privacy of each twin. Every research proposal must be approved by a panel to assure the scientific value of the project, the justification for doing the study in twins, and to ensure that the privacy and safety of individual twins will be protected.
Twin registries can be useful starting points for investigating many questions about the genetic and environmental determinants of a trait. Records linkage studies involve no personal contact with the twins. Instead, information in the twin registry is "linked" electronically to information in another database, such as a national health insurance database or a cancer registry. In this way, twins with a particular health problem can be identified, and concordance estimates can be calculated. Similarly, information collected for each twin at registration can later be used to investigate certain kinds of questions without ever contacting the individual twins. On the other end of the spectrum, twins may be asked to volunteer for physical examinations, blood tests, radiological studies, or interviews. Depending on the questions asked, such studies may be useful for comparing concordance, or for identifying risk factors or modifying factors for a trait.
Twin Studies to Investigate the Cause of Parkinson's Disease
An example of the use of investigations in twins to understand more about a disease is provided by recent work in Parkinson's disease. Parkinson's disease (PD) is a progressive neurodegenerative disease causing slowness, tremor, and problems with walking and balance. PD is rare before age fifty but becomes more common thereafter, with increasing age. The cause of PD has long been debated. Both genetic and environmental causes have been suggested, but neither has been definitively shown. Researchers turned to studies in twins to determine the relative contribution of genes and environment to the disease.
The first studies identified twin pairs by recruiting through physicians and PD patient organizations. Studies in the United States, the United Kingdom, and Germany identified 103 pairs, of which only thirteen were concordant for PD. In Finland, forty-two twins with PD were identified by records linkage, but among these was only one concordant pair—a DZ pair. No study had convincingly demonstrated greater monozygotic than dizygotic concordance for the disease, and in all studies the preponderance of twin pairs were discordant for disease. These findings supported an environmental cause of PD. Nonetheless, the advent of molecular genetics prompted great interest in investigations of genetic causes of disease and prompted the resurgence of the hypothesis that all PD had a genetic cause. To address this, a study in a large, unselected cohort—the National Academy of Sciences/National Resource Council (NAS/NRC) World War II Veteran Twins Registry—was undertaken.
In the mid-1950s, the Medical Follow-up Agency of the Institute of Medicine of the NAS/NRC established a registry of approximately 32,000 Caucasian male twins, all of whom were born between 1917 and 1927 and were veterans of the U.S. Armed Services. In all, 161 twin pairs were identified, twenty-one of which were concordant for PD, as shown in Table 2. In those few pairs with early-onset PD, concordance was greater in MZ pairs. In those with more typical PD, beginning after age fifty, there was no difference in MZ and DZ concordance.
These findings suggest a strong genetic determinant for early-onset disease but predominantly environmental causes in more typical late-onset disease. One caveat is the narrow age range of the twins, who were sixty-seven to seventy-seven years old when studied. Since PD is a late-life disorder, PD in some twins may have been missed with an examination at only one time point. To overcome this, a second evaluation is in progress.
Risk-Factor Investigations in Twins
Studies of twin pairs discordant for disease can be useful for identifying risk factors for disease. Since both genetic and environmental factors are extensively shared by twins, particularly by MZ twins, case-control studies can be particularly powerful. In such a study, each twin is interviewed with regard to specific environmental factors—such as occupation, lifestyle factors, illnesses or injuries, and diet—prior to the onset of the disease in the affected twin. The presence of these factors in the twin with the disease is compared to the twin without disease. An association of an environmental factor with the disease suggests this factor may be causally related. Factors more common among the unaffected twins suggest that the factor may protect against the development of the disease.
Environmental influences on PD have been investigated by studying discordant twin pairs. PD has repeatedly been found to be more common in people who do not smoke cigarettes. Some have proposed that some people are genetically predisposed to both Parkinson's disease and smoking, while others suggest cigarette smoking somehow prevents the degeneration that leads to PD. In two studies of discordant twin pairs, cigarette smoking was more common in the twin without Parkinson's disease, especially in the MZ pairs. Because monozygotic twins are genetically identical, this pattern tips the scales in favor of a direct biological action of cigarette smoke.
As medicine focuses more on early intervention or prevention, it becomes important to identify those persons at risk for a particular condition. This can be a problem if there is no diagnostic test. In discordant twin pairs, the unaffected twin is more likely to be "at risk" for a particular condition, whether due to shared genes or environment, than would be true for two nontwins. Therefore, studying the unaffected "at risk" twin may help to clarify what features are useful for predicting those who later will develop a particular disease. For example, in the PD twin study, the unaffected twins are being studied prospectively with brain imaging tests that may show early evidence of injury to the brain area damaged in PD. If abnormalities on this test are found to precede the development of PD, this could provide a useful method of early detection. When treatments to slow or stop the onset of PD are available, individuals with imaging abnormalities may receive intervention before symptoms develop.
Results from Twin Studies of Other Disorders and Conditions
The twin study method has been used to try to determine the extent of genetic or environmental influence on a wide variety of traits and conditions. Among these are sense of humor, which appears to be largely environmentally determined, as MZ and DZ pairs have similar concordance. Examples of other diseases in which MZ concordance exceeds DZ concordance, suggesting a significant genetic component, include addictive behaviors such as cigarette smoking and alcohol drinking, mental illnesses such as schizophrenia, as well as stroke and certain types of high blood pressure. Twin studies of many other disorders are ongoing.
Conclusion
Twin studies provide a unique approach to investigating the determinants of a disease or condition. A single twin study cannot absolutely determine the importance of genetic or environmental factors. However, the twin study method, in combination with other approaches, can be a powerful tool for unraveling the causes of disease.
Bibliography
Bouchard, T. J., et al. "The Sources of Human Psychological Differences: The Minnesota Study of Twins Reared Apart." Science 250 (1990): 223-228.
Segal, Nancy L. Entwined Lives: Twins and What They Tell Us about Human Behavior. New York: Plume, 2000.
Wright, Lawrence. Twins: And What They Tell Us about Who We Are. New York: John Wiley & Sons, 1997.
It has long been believed that there is a special relationship between identical twins, a belief that has become the subject of contemporary research from a variety of approaches. Research has suggested that there are startling correspondences between twins' temperaments, personalities, lifestyles, and even sensitivity to names.
In 1979, the University of Minnesota began a study of identical twins in which twins separated for years were investigated and subjected to medical and psychological tests. The results of nine identical twin studies, involving over 15,000 questions, demonstrated affinities between the subjects.
For example, unknown to each other, Jim Spring and Jim Lewis were raised in different Ohio towns. Both married and divorced women named Linda and chose women named Betty as second wives. Each of the two Jims named his son James Allan and had a favorite dog named Toy. Both twins had remarkable similarities in medical profiles, including identical blood pressures and sleep and heartbeat patterns. Both also suddenly put on 10 pounds at the same time in their lives. At the age of 18, both Jim twins suffered similar syndromes of intermittent migraine headaches. Their drinking and smoking habits were also identical, and both chewed their fingernails.
Another pair of identical twins, Jack and Oscar, were raised apart with completely different backgrounds. Jack was brought up as an American Jew by his father after his parents separated; the mother took Oscar back to Germany (where she had been born) where he was raised as a Catholic, later joining the Nazi Youth party. In adult life, Jack ran a store in San Diego, while Oscar became a factory supervisor in Germany. But both men wore wire-rimmed eyeglasses and mustaches and two-pocket shirts with epaulets. Both were absentminded and had other matching idiosyncracies, such as storing rubber bands on their wrists.
Bridget and Dorothy were identical British twins who were raised apart after being separated soon after birth, yet when they met in 1941, each wore two bracelets on one wrist, and a watch and bracelet on the other. Each sister also wore seven rings. Each twin had married and had a family of a boy and a girl. The sons had been christened Richard Andrew and Andrew Richard, while the daughters were Karen Louise and Catherine Louise.
Many such identical twins share IQ and psychological profiles, as well as EEG tracings. It is not yet clear whether the coincidences derive from some kind of psychic bonding or simply indicate some manifestation of inheritance. It should be noted that astrologers have investigated twins, with ambiguous results to date, with the idea of verifying and informing astrology.
Sources:
Watson, Peter. Twins: An Investigation Into the Strange Coincidences in the Lives of Separated Twins. London: Hutchinson,1981.
Twins have been mentioned since the beginning of recorded history, including ancient stories such as Romulus and Remus, the mythological founders of Rome, who were said to be twins abandoned and raised by a wolf. Although most mammals have large litters, primates including our species tend to have single offspring but occasionally have multiple births. Human twins are more common than people usually realize — about 1 in 85 births are twins. Surprisingly, as many as 20 per cent of all fetuses are twins but because of the hazards associated with twin pregnancies one member of the pair often dies very early in pregnancy.
Twins are of two types. One type, called identical twins, is derived from the same fertilized egg (called a zygote) that splits for unknown reasons to create two (sometimes more) genetically identical individuals. For about a third of identical twins, the zygote splits during the first five days after fertilization as it makes its way down to the womb. In this case, the identical twins have different sacs (called chorions) within the placenta. Two-thirds of the time, the zygote splits after it implants in the placenta and the twins share the same chorion. 'Siamese' twins are identical twins who split after about two weeks, timing that generally results in twins whose bodies are partially fused. Because identical twins come from the same zygote they are usually called monozygotic (MZ) in the scientific literature. The other type of twin involves two separate eggs that are fertilized by different sperm, just like any other brothers or sisters, and are called fraternal, non-identical, or dizygotic (DZ). About one-third of twins are identical twins. The identical twinning rate appears to be similar throughout the world and is not related to any demographic factors such as mother's age. In contrast, the fraternal twinning rate differs in different countries, and increases for older mothers, and may be heritable in some families. Fertility drugs have increased the rate of fraternal twinning by increasing the likelihood that more than one egg will be fertilized, as has in vitro fertilization in which several embryos are implanted in the hope that at least one will survive.
Twins are especially interesting today because of concerns about human cloning. Identical twins are more like clones than 'real' clones because identical twins spend their first nine months in the same womb and experience the same family environment and similar generational experiences. In contrast, if you were cloned from one of your cells, although your clone would be genetically identical to you, your clone would be raised in a different womb, different family, and different generation. For this reason, your clone would be less similar to you than if you had an identical twin to the extent that such environmental factors are important. Identical twins are so similar physically that it is often difficult to tell them apart, especially when they are young. For example, identical twins have the same eye colour, hair colour and texture, similar fingerprints, their heights are usually within an inch, and their weight is within a few pounds. In fact, with greater than 95 per cent accuracy, you can tell if twins are identical simply by asking if they are easily confused, because this single question summarizes many physical characteristics that are highly heritable. (DNA is used to assess identical twin status with nearly 100 per cent accuracy — if twins differ for any DNA markers they cannot be identical twins.) Fraternal twins can look similar, as siblings sometimes do, but they do not look identical and are not often confused.
Identical twins are not only similar on the outside. They are also similar physiologically and psychologically. For example, if one identical twin has epilepsy, the chances are greater than 90 per cent that the twin partner also has epilepsy. Identical twin concordances for some other medical illnesses are 70 per cent for ulcers, 60 per cent for Alzheimer's disease, 50 per cent for rheumatoid arthritis, and 30 per cent for ischemic heart disease. For a few diseases, however, identical twins are not very similar, such as chronic obstructive pulmonary disease (15 per cent), breast cancer (15 per cent), and Parkinson's disease (10 per cent), which strongly suggests that susceptibility to these diseases is not much inherited.
Identical twins are at least as similar psychologically. For example, concordance for identical twins is about 60 per cent for autism, 50 per cent for schizophrenia, and 40 per cent for major depressive disorder. This evidence for genetic influence is not limited to disorders but also extends to behavioural differences between people within the normal range of variation. For example, identical twins are nearly as similar to each other in intelligence as measured by IQ tests as are the same individuals tested twice. For personality, identical twins are somewhat less similar than for physical traits and intelligence. The correlation is a statistic describing resemblance that runs from 0.00 indicating no more than chance resemblance within pairs of twins to 1.0 indicating that members of a twin pair are exactly the same. Identical twins correlate about 0.90 for height, about 0.80 for weight, and about 0.80 for IQ scores, but only about 0.40 for most personality traits.
What about your clone? As mentioned above, your clone could be considerably less similar than your identical twin if growing up in the same environment is responsible for making identical twins similar because your clone would not experience so similar an environment. The issue is the extent to which environmental factors are responsible for the similarity of identical twins, which is the long-standing issue of nature (genetics) and nurture (environment). That is, it has long been known that family members resemble each other not just physically but also behaviourally. Do such traits run in families for reasons of nature or nurture? First described by Francis Galton in England in the late 19th century, the twin method compares similarity for identical and fraternal twins and has been the workhorse for disentangling nature and nurture. The twin method is the main reason why twins are so prominent in the scientific literature. The twin method is like an experiment in which one group (identical twins) are twice as similar genetically as another group (fraternal twins). If this twofold greater genetic similarity of identical twins has no effect on a trait, we would expect that identical twins would be no more similar than fraternal twins for the trait. Because half of fraternal twin pairs are opposite-sex twins, the experiment is better if same-sex fraternal twins are compared to identical twins, who are of course always of the same sex because they are genetically identical.
The twin method confirms, unsurprisingly, that individual differences in height are largely due to genetic differences: identical twins are highly similar for height (0.90) and fraternal twins are also just about as similar as expected on the basis of their genetic relatedness (0.45). You may be more surprised to learn that individual differences in weight are almost as highly heritable as height, with identical twin correlations of about 0.80 and fraternal twin correlations of about 0.40. The reason why the results for weight are more surprising than those for height is that you know that you can do more about your weight than you can do about your height. This raises an important point for interpreting results of twin studies. Finding evidence for genetic influence (often called heritability) does not refer to 'what could be': if you stop eating you will lose weight even though weight is highly heritable. Results of the twin method describe 'what is' in a particular sample rather than predicting what could be. That is, the twin method assesses the relative effects of genetic and environmental differences on observed differences in weight as they exist for a particular sample given that sample's genetic and environmental differences at that time. If you change the genetics of the sample (for example, by studying different ethnic groups) or if you change the environment (for example, by providing fattier foods), you could change the relative effects of genes and environments. This explanation is also relevant to why there is no paradox that twin results show high heritability for height and weight even though height and weight have both increased considerably on average in developed countries in recent generations.
Results of twin studies of physiological and psychological disorders and dimensions also indicate that the resemblance for identical twins discussed above is almost entirely due to genetics, not to environment. The results of the twin method comparing identical and fraternal twins are supported by results from a few hundred pairs of identical twins reared apart which generally suggest that identical twins reared apart are nearly as similar as identical twins reared for psychological as well as physical traits. Thus, we can safely conclude that your clone would be almost as similar to you as would an identical twin. For some traits this means that your clone would be very similar to you --for height and weight and general physical appearance, susceptibility to certain diseases such as epilepsy and ulcers, and intelligence. For a few traits such as Parkinson's disease and breast cancer, there would be little resemblance. For most disorders and dimensions, however, your clone (or identical twin) would be moderately similar to you but would not be just a chip off the same old block. Moreover, research on twins suggests that despite their similarity there is no evidence that identical twins suffer from having a clone either physically or psychologically. For example, identical twins are just as intelligent and as well adjusted as non-twin individuals. Identical twins are clones and they are doing fine.
(Published 2004)
— Robert Plomin
Bibliography
Plomin, R., DeFries, J. C., McClearn, G. E., and McGuffin, P. (2001). Behavioral Genetics (4th edn.).
Segal, N. (1999). Entwined Lives: Twins and What they Tell us about Human Behavior.
The appearance in a dream of two of a kind (people or animals) may symbolize the union of opposites, or a duality of consciousness, either in harmony with, or in conflict between, ideas and decisions. The dreamer may be demonstrating two distinct personalities.
Twins are two offspring resulting from the same pregnancy, usually born in close succession. They can be the same or different sex. Twins can either be monozygotic (MZ, colloquially "identical") or dizygotic (DZ, colloquially "fraternal" or "non-identical").
The general term for more than one offspring in the same pregnancy (multiple birth) is multiples; a fetus which develops alone in the womb is called a singleton.
Statistics
The number of living human twins in the world has been estimated to be approximately 125 million in 2006[1] (roughly 1.9% of the world population), with just 10 million monozygotic twins (roughly 0.2% of the world population and 8% of all twins). The twin birth rate in the United States in 2004, 2005 and 2006 was slightly above 32 twin live births per 1,000 live births[2].
Due to the limited size of the mother's womb, multiple pregnancies are much less likely to carry to full term than singleton births, with twin pregnancies lasting only 37 weeks (3 weeks less than full term) on average.[3] Since premature births can have health consequences for the babies, twin births are often handled with special precautions.
The Yoruba, a large west African ethnic group, have the highest rate of twinning in the world, at 45 twins per 1,000 live births.[4][5][6] Some researchers have claimed this may be because of high consumption of a specific type of yam, Dioscorea rotundata or white yam containing a natural hormonephytoestrogen which may stimulate the ovaries to release an egg from each side.[7][8]
Types of twins
There are five common variations of twinning. The three most common variations are all dizygotic:
Male–female twins are the most common result, 50 percent of DZ twins and the most common grouping of twins.
Female DZ twins (sometimes called sororal twins)
Male DZ twins
The other two variations are monozygotic twins:
Female MZ twins
Male MZ twins (least common)
Among non-twin births, male singletons are slightly (about five percent) more common than female singletons. There are also the mirror image variations: this is where the twins develop reverse asymmetric features. The rates for singletons vary slightly by country. For example, the sex ratio of birth in the US is 1.05 males/female,[9] while it is 1.07 males/female in Italy.[10] However, males are also more susceptible than females to death in utero, and since the death rate in utero is higher for twins, it leads to female twins being more common than male twins.
Another variety of twins, "polar body twins," is a phenomenon that was hypothesized to occur and may recently have been proven, very rarely, to exist. This would occur when a portion of a mature egg separates from itself. This is known as the first polar body, and it carries all the same genetic information as the egg. If polar body twins are fact, they would occur when two sperm fertilize both the egg and the first polar body. Generally the first polar body disintegrates. Polar body twinning would result in "half-identical" twins.[11]
Eight month old dizygotic twin girls napping
Dizygotic twins
Dizygotic twins (commonly known as fraternal twins, but also referred to as non-identical twins or biovular twins) usually occur when two fertilized eggs are implanted in the uterine wall at the same time. When two eggs are independently fertilized by two different sperm cells, DZ twins result. The two eggs, or ova, form two zygotes, hence the terms dizygotic and biovular.
Dizygotic twins, like any other siblings, have an extremely small chance of having the same chromosome profile. Like any other siblings, DZ twins may look similar, particularly given that they are the same age. However, DZ twins may also look very different from each other. They may be of different sexes or the same sex. The same holds true for brothers and sisters from the same parents, meaning that DZ twins are simply brothers and/or sisters who happen to have the same age.
Studies show that there is a genetic basis for DZ twinning. However, it is only their mother that has any effect on the chances of having DZ twins; there is no known mechanism for a father to cause the release of more than one ovum. Dizygotic twinning ranges from six per thousand births in Japan (similar to the rate of monozygotic twins) to 14 and more per thousand in some African countries.[7]
DZ twins are also more common for older mothers, with twinning rates doubling in mothers over the age of 35.[12] With the advent of technologies and techniques to assist women in getting pregnant, the rate of fraternals has increased markedly. For example, in New York City's Upper East Side there were 3,707 twin births in 1995; there were 4,153 in 2003; and there were 4,655 in 2004. Triplet births have also risen, from 60 in 1995 to 299 in 2004.
Monozygotic twins
Comparison of zygote development in monozygotic and dizygotic twins. In the uterus, a majority of monozygotic twins (60–70%) share the same placenta but have separate amniotic sacs. In 18–30% of monozygotic twins each fetus has a separate placenta and a separate amniotic sac. A small number (1–2%) of monozygotic twins share the same placenta and amniotic sac. Dizygotic twins each have their own placenta and own amniotic sac.
Various types of chorionicity and amniosity (how the baby's sac looks) in monozygotic (one egg/identical) twins as a result of when the fertilized egg divides
Monozygotic twins, frequently referred to as identical twins, occur when a single egg is fertilized to form one zygote (monozygotic) which then divides into two separate embryos. When the division of the developing zygote into 2 embryos occurs, 99% of the time it is within 8 days of fertilization. This results in monozygotic twins. If the division of the zygote occurs later than the 8 days then conjoined twins are usually the result. Monozygotic twins are the same sex and their traits and physical appearances are very similar but not exactly the same; although they have nearly identical DNA,[13] environmental conditions both inside the womb and throughout their lives influence the switching on and off of various genes. Division of the zygote into two embryos is not considered to be a hereditary trait, but rather an anomaly that occurs in birthing at a rate of about three in every 1000 deliveries worldwide,[14] regardless of race. The two embryos develop into fetuses sharing the same womb. When one egg is fertilized by one sperm cell, and then divides and separates, two identical cells will result. If the zygote splits very early (in the first two days after fertilization), each cell may develop separately its own placenta (chorion) and its own sac (amnion). These are called dichorionic diamniotic (di/di) twins, which occurs 18–36% of the time.[15] Most of the time in MZ twins the zygote will split after two days, resulting in a shared placenta, but two separate sacs. These are called monochorionic diamniotic (mono/di) twins, occurring 60–70% of the time.[15]
In about 1–2% of MZ twinning the splitting occurs late enough to result in both a shared placenta and a shared sac called monochorionic monoamniotic (mono/mono) twins.[15] Finally, the zygote may split extremely late, resulting in conjoined twins. Mortality is highest for conjoined twins due to the many complications resulting from shared organs. Mono/mono twins have an overall in-utero mortality of about 50 percent, principally due to cord entanglement prior to 32 weeks gestation. If expecting parents choose hospitalization, mortality can decrease through consistent monitoring of the babies. Hospitalization can occur beginning at 24 weeks, but doctors prefer a later date to prevent any complications due to premature births. The choice is up to the parents when to start hospitalization. Many times, monoamniotic twins are delivered at 32 weeks electively for the safety of the babies. In higher order multiples, there can sometimes be a combination of DZ and MZ twins.
Mono/di twins have about a 25 percent mortality due to twin-to-twin transfusion syndrome. Di/di twins have the lowest mortality risk at about 9 percent, although that is still significantly higher than that of singletons. [16]
Monozygotic twins are genetically identical (unless there has been a mutation during development) and they are always the same sex. On rare occasions, monozygotic twins may express different phenotypes, normally due to an environmental factor or the deactivation of different X chromosomes in monozygotic female twins, and in some extremely rare cases, due to aneuploidy, twins may express different sexual phenotypes, normally due to an XXY Klinefelter's syndrome zygote splitting unevenly[17][18]). Monozygotic twins look alike, although they do not have the same fingerprints (which are environmental as well as genetic). As they mature, MZ twins often become less alike because of lifestyle choices or external influences. Genetically speaking, the children of MZ twins are half-siblings rather than cousins. If each member of one set of MZ twins reproduces with one member of another set of MZ twins then the resulting children would be genetic full siblings. It is estimated that there are around 10 million monozygotic twins and triplets in the world.
The likelihood of a single fertilisation resulting in MZ twins appears to be a random event, not a hereditary trait, and is uniformly distributed in all populations around the world.[12] This is in marked contrast to DZ twinning, which ranges from about six per thousand births in Japan (almost similar to the rate of MZ twins, which is around 4–5) to 15 and more per thousand in some parts of India[19] and up to 24 in the US, which might mainly be due to IVF (in vitro fertilisation). The exact cause for the splitting of a zygote or embryo is unknown.
Monozygotic twins have nearly identical DNA, but differing environmental influences throughout their lives affect which genes are switched on or off. This is called epigenetic modification. A study of 80 pairs of human twins ranging in age from three to 74 showed that the youngest twins have relatively few epigenetic differences. The number of epigenetic differences between MZ twins increases with age. Fifty-year-old twins had over three times the epigenetic difference of three-year-old twins. Twins who had spent their lives apart (such as those adopted by two different sets of parents at birth) had the greatest difference.[20] However, certain characteristics become more alike as twins age, such as IQ and personality.[21][22] This phenomenon illustrates the influence of genetics in many aspects of human characteristics and behaviour.
A recent theory posits that monozygotic twins are formed after a blastocyst essentially collapses, splitting the progenitor cells (those that contain the body's fundamental genetic material) in half. That leaves the same genetic material divided in two on opposite sides of the embryo. Eventually, two separate fetuses develop. The research was presented at a meeting of the European Society of Human Reproduction and Embryology in Lyon, France. Utilizing computer software to take photos every two minutes of 33 embryos growing in a laboratory, Dr. Dianna Payne, a visiting research fellow at the Mio Fertility Clinic in Japan, documented for the first time the early days of twin development. Payne also discovered explanation for why in-vitro fertilization techniques are more likely to create twins. Only about three pairs of twins per 1,000 deliveries occur as a result of natural conception, while for IVF deliveries, there are nearly 21 pairs of twins for every 1,000.[23][24]
Zygosity, chorionicity and amniocity
The two types of twins discussed before, monozygotic and dizygotic, are generally referred to as zygocity. Zygocity reflects the genetic type of twins. In contrast, the chorionity and amniocity describe degree of shared environment of the twins in the womb. Chorionity refers to the number of chorionic sacs, while amniocity refers to the number of amniotic sacs. Monochorionic twins share the same placenta, and monoamniotic twins also share the same amniotic sac. Monoamniotic twins are always monozygotic.[25]Monochorionic twins, on the other hand, may be dizygotic. Furthermore, two placentas (dichorionic twins) does not necessarily exclude monozygocity since monozygotic twins can have two placentas. Chorionicity and amniocity are a result of the division time. Dichorionic twins divide within the first 4 days. Monoamnionic twins divide after the first week.
Demographics
A recent study found that vegan mothers are five times less likely to have twins than vegetarian or omnivore mothers, and concluded that "Genotypes favoring elevated IGF and diets including dairy products, especially in areas where growth hormone is given to cattle, appear to enhance the chances of multiple pregnancies due to ovarian stimulation."[26]
From 1980–97, the number of twin births in the United States rose 52%.[27] This rise can at least partly be attributed to the increasing popularity of fertility drugs like Clomid and procedures such as in vitro fertilization, which result in multiple births more frequently than unassisted fertilizations do. It may also be linked to the increase of growth hormones in food.[26]
Ethnicity
About 1 in 90 human births (1.1%) results from a twin pregnancy.[28] The rate of dizygotic twinning varies greatly among ethnic groups, ranging as high as about 45 per 1000 births for the Yoruba to 10% for Linha São Pedro, a tiny Brazilian settlement which belongs to the city of Cândido Godói.[29] In Cândido Godói, one in five pregnancies have resulted in twins – most of them blond haired and blue eyed.[30] The Argentinian historian Jorge Camarasa has put forward a theory that experiments of the Nazi doctor Joseph Mengele could be responsible for the high ratio of twins in the area.[31] His theory was rejected by Brazilian scientists who had studied twins living in Linha São Pedro; they suggested genetic factors within that community as a more likely explanation.[32]
The widespread use of fertility drugs causing hyperovulation (stimulated release of multiple eggs by the mother) has caused what some call an "epidemic of multiple births". In 2001, for the first time ever in the US, the twinning rate exceeded 3% of all births. Nevertheless, the rate of monozygotic twins remains at about 1 in 333 across the globe.
In a study on the maternity records of 5750 Hausa women living in the Savannah zone of Nigeria, there were 40 twins and 2 triplets/1000 births. Twenty six per cent of twins were monozygous. The incidence of multiple births, which was about five times higher than that observed in any western population, was significantly lower than that of other ethnic groups, who live in the hot and humid climate of the southern part of the country. The incidence of multiple births was related to maternal age but did not bear any association to the climate or prevalence of malaria.[33]
Predisposing factors
The predisposing factors of monozygotic twinning are unknown.
Dizygotic twin pregnancies are slightly more likely when the following factors are present in the woman:
She has a family history of dizygotic twinning, especially a mother who is a twin.[citation needed]
Women undergoing certain fertility treatments may have a greater chance of dizygotic multiple births. This can vary depending on what types of fertility treatments are used. With in vitro fertilisation (IVF), this is primarily due to the insertion of multiple embryos into the uterus. Some other treatments such as the drug Clomid can stimulate a woman to release multiple eggs, allowing the possibility of multiples. Many fertility treatments have no effect on the likelihood of multiple births.[citation needed]
Delivery interval
A 15-year German study[34] of 8,220 vaginally delivered twins (that is, 4,110 pregnancies) in Hesse yielded a mean delivery time interval of 13.5 minutes.[35] The delivery interval between the twins was measured as follows:
Within 15 minutes: 75.8%
16-30 minutes: 16.4%
31-45 minutes: 4.3%
46-60 minutes: 1.7%
Over 60 minutes: 1.8% (72 instances)
The study stated that the occurrence of complications "was found to be more likely with increasing twin-to-twin delivery time interval" and suggested that the interval be kept short, though it noted that the study did not examine causes of complications and did not control for factors such as the level of experience of the obstetrician, the wish of the women giving birth, or the "management strategies" of the procedure of delivering the second twin.
Researchers suspect that as many as 1 in 8 pregnancies start out as multiples, but only a single fetus is brought to full term, because the other has died very early in the pregnancy and has not been detected or recorded.[36] Early obstetric ultrasonography exams sometimes reveal an "extra" fetus, which fails to develop and instead disintegrates and vanishes. This is known as vanishing twin syndrome.
Conjoined twins (or the deprecated term "Siamese twins") are monozygotic twins whose bodies are joined together during pregnancy. This occurs where the single zygote of MZ twins fails to separate completely, and the zygote starts to split after day 12[37] following fertilization. This condition occurs in about 1 in 50,000 human pregnancies. Most conjoined twins are now evaluated for surgery to attempt to separate them into separate functional bodies. The degree of difficulty rises if a vital organ or structure is shared between twins, such as the brain, heart or liver.
A chimera is an ordinary person or animal except that some of their parts actually came from their twin or from the mother. A chimera may arise either from monozygotic twin fetuses (where it would be impossible to detect), or from dizygotic fetuses, which can be identified by chromosomal comparisons from various parts of the body. The number of cells derived from each fetus can vary from one part of the body to another, and often leads to characteristic mosaicism skin colouration in human chimeras. A chimera may be intersex, composed of cells from a male twin and a female twin. In addition, in certain cases the person or chimera may have two sets of DNA.
Sometimes one twin fetus will fail to develop completely and continue to cause problems for its surviving twin. One fetus acts as a parasite towards the other. Sometimes the parasitic twin becomes an almost indistinguishable part of the other, and sometimes this needs to be medically dealt with.
Partial molar twins
A very rare type of parasitic twinning is one where a single viable twin is endangered when the other zygote becomes cancerous, or molar. This means that the molar zygote's cellular division continues unchecked, resulting in a cancerous growth that overtakes the viable fetus. Typically, this results when one twin has either triploidy or complete paternal uniparental disomy, resulting in little or no fetus and a cancerous, overgrown placenta, resembling a bunch of grapes.
Miscarried twin
Occasionally, a woman will suffer a miscarriage early in pregnancy, yet the pregnancy will continue; one twin was miscarried but the other was able to be carried to term. This occurrence is similar to the vanishing twin syndrome, but typically occurs later than the vanishing twin syndrome.
Low birth weight
Twins typically suffer from the lower birth weights and greater likelihood of prematurity that is more commonly associated with the higher multiple pregnancies. Throughout their lives twins tend to be smaller than singletons on average.
Monozygotic twins who share a placenta can develop twin-to-twin transfusion syndrome. This condition means that blood from one twin is being diverted into the other twin. One twin, the 'donor' twin, is small and anemic, the other, the 'recipient' twin, is large and polycythemic. The lives of both twins are endangered by this condition.
Twin studies are utilized in an attempt to determine how much of a particular trait is attributable to either genetics or environmental influence. These studies compare monozygotic and dizygotic twins for medical, genetic, or psychological characteristics to try to isolate genetic influence from epigenetic and environmental influence. Twins that have been separated early in life and raised in separate households are especially sought-after for these studies, which have been used widely in the exploration of human nature. However, the utility and accuracy of these twin studies has been called into question and remains controversial. Classical twin studies have largely been replaced in favor of modern molecular genetic methodologies.
Unusual twinnings
There are some patterns of twinning that are exceedingly rare: while they have been reported to happen, they are so unusual that most obstetricians or midwives may go their entire careers without encountering a single case.[citation needed]
Among dizygotic twins, in rare cases, the eggs are fertilized at different times with two or more acts of sexual intercourse, either within one menstrual cycle (superfecundation) or, even more rarely, later on in the pregnancy (superfetation). This can lead to the possibility of a woman carrying fraternal twins with different fathers (that is, half-siblings). This phenomenon is known as heteropaternal superfecundation. One 1992 study estimates that the frequency of heteropaternal superfecundation among dizygotic twins whose parents were involved in paternity suits was approximately 2.4%; see the references section, below, for more details.
Dizygotic twins from biracial couples can sometimes be mixed twins, which exhibit differing ethnic and racial features. One such pairing was born in Germany in 2008 to a white father from Germany and a black mother from Ghana.[38]
Heterotopic pregnancy is an exceedingly rare type of dizygotic twinning in which one twin implants in the uterus as normal and the other remains in the fallopian tube as an ectopic pregnancy. Ectopic pregnancies must be resolved because they can be life-threatening to the mother. However, in most cases, the intrauterine pregnancy can be salvaged. Heterotopic pregnancy occurs at a rate of about one in 30,000 pregnancies.[citation needed]
Among monozygotic twins, in extremely rare cases, twins have been born with opposite sexes (one male, one female). The probability of this is so vanishingly small (only 3 documented cases[39]) that multiples having different sexes is universally accepted as a sound basis for a clinical determination that in utero multiples are not monozygotic. When monozygotic twins are born with different sexes it is because of chromosomal birth defects. In this case, although the twins did come from the same egg, it is incorrect to refer to them as genetically identical, since they have different karyotypes.
Monozygotic twins can develop differently, due to different genes being activated.[40] More unusual are "semi-identical twins". These "half-identical twins" are hypothesized to occur when an unfertilized egg cleaves into two identical attached ova and which are viable for fertilization. Both cloned ova are then fertilized by different sperm and the coalesced eggs undergo further cell duplications developing as a chimeric blastomere. If this blastomere then undergoes a twinning event, two embryos will be formed, each of which have different paternal genes and identical maternal genes.
This results in a set of twins with identical genes from the mother's side, but different genes from the father's side. Cells in each fetus carry genes from either sperm, resulting in chimeras. This form had been speculated until only recently being recorded in western medicine.[41][42][43]
Twins are common in many animal species, such as cats, sheep, ferrets and deer. The incidence of twinning among cattle is about 1–4%, and research is under way to improve the odds of twinning, which can be more profitable for the breeder if complications can be sidestepped or managed. The nine-banded armadillo (Dasypus novemcinctus) has identical twins (usually four babies) as its regular reproduction and not as exceptional cases.[44][45]
^ Martin, Joyce A.; Hamilton, Brady E.; Sutton, Paul D.; Ventura, Stephanie J.; Menacker, Fay; Kirmeyer, Sharon and Mathews, T.J. Births: Final Data for 2006, National Vital Statistics Report, Vol. 57, No. 7, Jan. 7, 2009, 102 pp. (Table 39, pp.83-84).
^ Elliott, JP (December 2007). "Preterm labor in twins and high-order multiples.". Clinical Perinatology34 (4): 599–609. doi:10.1016/j.clp.2007.10.004. PMID 18063108. "Unlike singleton gestation where identification of patients at risk for PTL is often difficult, every multiple gestation is at risk for PTL, so all patients can be managed as being at risk.".
^ ab O. Bomsel-Helmreich; W. Al Mufti (1995). "The mechanism of monozygosity and double ovulation". in Louis G. Keith, Emile Papierik, Donald M. Keith and Barbara Luke. Multiple Pregnancy: Epidemiology, Gestation & Perinatal Outcome. Taylor and Francis. pp. 34. ISBN 1-85070-666-2.
^ ab Bortolus, Renata; Fabio Parazzini, Liliane Chatenoud, Guido Benzi, Massimiliano Maria Bianchi, Alberto Marini (1999). "The epidemiology of multiple births". Human Reproduction Update (European Society of Human Reproduction and Embryology) 5 (2): 179–187. doi:10.1093/humupd/5.2.179. ISSN1355-4786. PMID 10336022.
^Benirschke, Kurt (2004). "Multiple Gestation". in Robert Resnik, Robert K. Creasy and Jay D. Iams. Maternal-Fetal Medicine: Principles and Practice (5th ed.). Philadelphia: W. B. Saunders Company. pp. 55–62. ISBN 0-7216-0004-2.
^ Matte, U; MG Le Roux, B Bénichou, JP Moisan and R Giugliani (1996). "Study on possible increase in twinning rate at a small village in south Brazil". Acta Genet Med Gemellol (Roma)45 (4): 431–437. PMID 9181177.
^ "Twin-to-twin delivery time interval: influencing factors and effect on short-term outcome of the second twin"; Werner Stein, Björn Misselwitz & Stefan Schmidt; Acta Obstetricia et Gynecologica Scandinavica, 87(3):346-353, 2008.
^ There were 836,104 deliveries of babies in Hesse over the 15-year study period, including 11,740 twin pregnancies, of which only 4,110 met the inclusion criteria and hence were examined in the study. The excluded twin pregnancies were in cases of (1) delivery before 34 weeks of gestation; (2) when the first twin was delivered by caesarean section; (3) when either of the twins had died in the womb before the onset of labor; and (4) when the pregnancy had been complicated by fetal malformations or Twin-to-twin transfusion syndrome.
^ Gedda, Luigi (1995). "The role of research in twin medicine". in Louis G. Keith, Emile Papiernik, Donald M. Keith and Barbara Luke. Multiple Pregnancy: Epidemiology, Gestation & Perinatal Outcome. Taylor and Francis. pp. 4. ISBN 1-85070-666-2.
^ Shulman, Lee S.; Vugt, John M. G. van (2006). Prenatal medicine. Washington, DC: Taylor & Francis. pp. Page 447. ISBN 0-8247-2844-0.
^ Souter, Vivienne L.; Melissa A. Parisi, Dale R. Nyholt, Raj P. Kapur, Anjali K. Henders, Kent E. Opheim, Daniel F. Gunther, Michael E. Mitchell, Ian A. Glass and Grant W. Montgomery (April 2007). "A case of true hermaphroditism reveals an unusual mechanism of twinning". Hum. Genet.121 (2): 179–85. doi:10.1007/s00439-006-0279-x. PMID 17165045.
Nieuwint, Aggie; Rieteke Van Zalen-Sprock, Pieter Hummel, Gerard Pals, John Van Vugt, Hans Van Der Harten, Yvonne Heins and Kamlesh Madan (1999). "'Identical' twins with discordant karyotypes". Prenatal Diagnosis19 (1): 72–6. doi:10.1002/(SICI)1097-0223(199901)19:1<72::AID-PD465>3.0.CO;2-V. PMID 10073913.
Wenk RE, Houtz T, Brooks M, Chiafari FA (1992). "How frequent is heteropaternal superfecundation?". Acta geneticae medicae et gemellologiae41 (1): 43–7. PMID 1488855.
Girela, Eloy; Jose A. Lorente, J. Carlos Alvarez, Maria D. Rodrigo, Miguel Lorente and Enrique Villanueva (1997). "Indisputable double paternity in dizygous twins". Fertility and Sterility67 (6): 1159–61. doi:10.1016/S0015-0282(97)81456-2. PMID 9176461.
Schein, Elyse; Paula Bernstein (2007). Identical Strangers: A Memoir of Twins Separated and Reunited. New York: Random House. ISBN 1-4000-6496-1. OCLC123390922.
Dansk (Danish)
n. - tvilling, mage adj. - tvillinge- v. intr. - passe sammen med v. tr. - knytte, parre
idioms:
twin bed to enkeltsenge
twin cam dobbelt knastaksel
twin set cardigansæt
twin town venskabsby
Nederlands (Dutch)
tweeling, dubbel
Français (French) n. - jumeau, chambre à deux lits, pendant/pareil (un de deux objets) adj. - jumeau, jumelé, double v. intr. - jumeler v. tr. - jumeler
idioms:
twin bed lit jumeau
twin set twin-set
twin town ville jumelée
Deutsch (German) n. - Zwilling v. - paaren, Zwillinge zur Welt bringen adj. - Zwillings-
idioms:
twin bed eins von zwei gleichen Einzelbetten
twin set Twinset (Pullover u. Jacke v. gleichem Material u. Farbe)
twin town Partnerstadt
Ελληνική (Greek) n. - δίδυμος (αδελφός) v. - συνταιριάζω, ζευγαρώνω, αδελφοποιώ adj. - δίδυμος
idioms:
twin bed δίδυμο κρεβάτι
twin cam διαξονικός
twin set ανσάμπλ μπλούζας-ζακέτας, φούστα-μπλούζα
twin town αδελφοποιημένη πόλη
Italiano (Italian) gemello
idioms:
twin bed letto ad una piazza
twin cam doppia camma
twin set twinset
twin town cittý gemellata
Português (Portuguese) n. - gêmeo (m) v. - igualar, dar à luz gêmeos adj. - duplo
idioms:
twin bed camas separadas
twin cam câmara dupla
twin set conjunto de gêmeos
twin town cidade irmã
Русский (Russian) близнецы
idioms:
twin bed двойная постель
twin cam двойная скрепка
twin set двойной комплект женского платья
twin town города-близнецы
Español (Spanish) n. - gemelo, mellizo, antena de dos hilos adj. - gemelo, doble, de dos v. intr. - parir gemelos, nacer gemelo v. tr. - hacer igual o gemelo, juntar, emparejar, unir (dos cosas)
idioms:
twin bed camas gemelas
twin set conjunto
twin town ciudad gemelo
Svenska (Swedish) n. - tvilling, pendang, motstycke, make v. - föda tvillingar, para ihop, koppla samman, förena, passa ihop adj. - tvilling-, par-, dubbel-, exakt likadan
עברית (Hebrew) n. - תאום adj. - תאומי, זהה v. intr. - ילדה תאומים, קשר קשרים עם עיר תאומה v. tr. - ילדה תאומים, זיווג, הביא ליצירת קשרים עם עיר תאומה
If you are unable to view some languages clearly, click here.
To select your translation preferences click here.
