multiple birth

 

bringing forth of more than one offspring at birth. Although many smaller mammals bear several young at a time, multiple births are relatively uncommon in humans and other primates. Twinning, the process that leads to the production of more than one offspring, results in twins, and with decreasing frequency, triplets, quadruplets, quintuplets, sextuplets, septuplets, and octuplets.

In the one-egg, or identical, type of twinning, a single fertilized ovum divides to form two complete organisms. Such twins are always of the same sex, are usually extraordinarily similar in physical appearance, and have identical blood-group types. Twinning to form one-egg identical twins usually takes place early in pregnancy. If considerable development has taken place before the twinning occurs, there may be an incomplete separation of the two embryos resulting in conjoined offspring called Siamese twins.

Fraternal twins, which are more common than identical twins, are those that develop from two separate ova, each fertilized by a sperm. Fraternal twins may or may not be of the same sex and need not resemble each other more than do any other two offspring of the same parents. In the United States twins occur once in approximately 40 births. In rare cases, non-identical embryos can fuse in the womb to produce a condition called chimerism, in which some of the individual's cells come from one of the embryos and the rest of the cells come from the other, genetically distinct embryo.

The incidence of multiple-egg births is in part genetically determined, varying according to race and family tendencies; and it is also influenced by external factors, i.e., the incidence increases with increasing age of the mother and the number of children she has already borne. One-egg, or identical, twinning occurs with the same frequency in all women, regardless of race, age, or other factors. There is evidence from comparative biology that deleterious factors in the environment of the newly fertilized ovum, such as a reduction in oxygen, increase the likelihood of one-egg twinning. Fertility drugs such as clomiphene, which are used when the cause of infertility is lack of released ova, sometimes cause several ova to be released and fertilized simultaneously. The use of these drugs has led to a rise in the incidence of multiple births, including sextuplets, septuplets, and octuplets.

Quadruplet, quintuplet, etc. redirect here. For the musical use of the terms, see tuplet.

A multiple birth occurs when more than one fetus is carried to term in a single pregnancy. Different names for multiple birth are utilized, depending on the number of offspring. Common multiples are two and three, known as twins and triplets respectively. Twins, triplets and other multiple births occur to varying degrees in most animal species, although the term is most applicable to placental species.

There are two common types of multiple births, fraternal (dizygotic) and identical (monozygotic). Identical siblings arise where one egg is fertilised and the resulting zygote splits into more than one embryo. Identical siblings therefore have the same genetic material. Fraternal siblings result from the fertilisation and implantation of more than one egg, so fraternal siblings are not genetically identical, and instead have the coequal genetic similarity any other full siblings do. In some multiple births, it is possible for a combination of these (for example, a triplet may have one fraternal baby from one egg, plus two identical twins from the other). This is called a polyzygotic birth.

A very rare third type of twinning is believed to occur in which an unfertilized ovum splits into two identical cells prior to fertilization, creating a second complete ovum, called the "polar body". After splitting, both ova are then fertilized by different sperm. This results in twins who have received identical genes from their mother, but of the genetic material they received from their father, only about half of it is shared. Thus polar body twins have about 75% of their genes in common, less than the 100% shared by identical twins but more than the 50% shared by fraternal twins. They share some features of identical twins and some features of fraternal twins and are so-called half-identical twins. However, DNA-based zygosity tests are currently not available to determine if twins are polar-body twins, so there has been no confirmation that such twins actually exist.

The most common form of human multiple birth is twins (two babies), but the typical order of multiple births in other placental species differs dramatically. Some species give birth to multiple offspring as a matter of course and the resulting group of offspring is called a litter.

Terminology

• Monozygotic — multiple (typically two) fetuses produced by the splitting of a single zygote
• Dizygotic — multiple (typically two) fetuses produced by two zygotes
• Polyzygotic — multiple fetuses produced by two or more zygotes
• Litter — the offspring produced by a multiple birth in non-human placentals.

Terms used for the order of multiple births are largely derived from the Latin names for numbers. Two offspring (twins) is the most common form of multiple births to seven offspring (septuplets) being the rarest form of multiple births.

• Two offspring — twins
• Three offspring — triplets
• Four offspring — quadruplets
• Five offspring — quintuplets
• Six offspring — sextuplets
• Seven offspring — septuplets
• Eight offspring — octuplets
• Nine offspring — nonuplets
• Ten offspring — decaplets
• Eleven offspring — undecaplets
• Twelve offspring — duodecaplets
• Thirteen offspring — tredecaplets
• Fourteen offspring — quattrodecaplets
• Fifteen offspring — quindecaplets

Higher order multiples

High orders of multiple births (three or more offspring in one birth) may result in a combination of fraternal (genetically different) and identical (genetically identical) siblings. The latter are also called super twins. For example, a set of quadruplets may consist of two sets of identical twins. In such a case each child has one identical and two fraternal siblings. Identical triplets or quadruplets are very rare, as this means that the original fertilized egg split and then one of the resultant cells splits again (producing identical triplets) or, even more rarely, a further split occurs (producing identical quadruplets). Sometimes the original fertilized egg will split twice (to produce four embryos), and all four may survive and produce quadruplets, or one of the embryos may not survive and result in identical triplets.

Human multiple births

The most common form of human multiple birth is twins (two babies), but cases of higher orders up to octuplets (eight babies) have all been recorded with all siblings being born alive. The largest set in which all members survived more than a few days is septuplets, the first of which was in 1997. The largest set to have even a single member survive is octuplets, in 1998 (with the seven surviving octuplets born in Texas).

There have been a few sets of nonuplets (nine) in which a few babies were born alive, though none lived longer than a few days. There have been cases of human pregnancies that started out with ten, eleven, twelve or fifteen fetuses, but no known instances of live births of such high multiples in a single pregnancy. Most of these pregnancies are the result of fertility medications and assisted reproductive technology (ART), though a set of duodecaplets (twelve) was conceived spontaneously (without the aid of fertility treatments) in Argentina in 1992.

Multiple pregnancies in humans are usually delivered before the full term of 40 weeks gestation. The average length of pregnancy is around 36 weeks for twins, 34 weeks for triplets and 32 weeks for quadruplets.

Causes and frequency

Human multiple births can occur either naturally (the woman ovulates multiple eggs or the fertilized egg splits into two) or as the result of infertility treatments such as IVF (several embryos are usually implanted to compensate for their lower viability) or fertility drugs (which can induce multiple fertile eggs being produced in one ovulatory cycle).

In general, twins occur naturally at approximately the rate of 1/89 of singleton births, triplets at 1/89 the rate of twin births, and so on (Hellin's Law). However, for reasons that are unknown, the older a woman is, the more likely she is to naturally have a multiple birth. It is theorized^{[citation needed]} that this is due to the higher follicle-stimulating hormone that older women sometimes have as their ovaries respond more sluggishly to FSH stimulation.

In North America, dizygotic twinning occurs about once in 83 conceptions and triplets about once in 8000 conceptions. A traditional approximation of the incidence of multiple pregnancies is as follows: ^[1]

• Twins 1:80
• Triplets 1:80² = 1:6400
• Quadruplets (Etc) 1:80³ = 1:512,000^[1]

The number of multiple births has increased over the last decades. For example, in Canada, between 1979 and 1999, the number of multiple birth babies increased 35%. Before the advent of ovulation-stimulating drugs, triplets were quite rare (approximately 1 in 8000 births) and higher order births so rare as to be almost unheard of.^{[citation needed]} Much of the increase can probably be attributed to the impact of fertility treatments, such as in-vitro fertilization. Younger patients who undergo treatment with fertility medication containing artificial FSH, followed by intrauterine insemination, are particularly at risk for multiple births of higher order.

Certain factors appear to increase the likelihood that a woman will naturally conceive multiples. These factors include:

• mother's age — women over 35 are more likely to have multiples than younger women
• mother's use of fertility drugs — approximately 35% of pregnancies arising through the use of fertility treatments such as IVF involve more than one child

The increasing use of fertility drugs as well as the increasing life expectancy for women have contributed to the rise in the rate of multiples over the last fifty years. Better nutrition also increases the likelihood of multiple births.

Risks

Recent increases over the last few years in the number of multiple births have also provoked concern over the risks to the fetus and to the mother.

The greater the number of babies in a single pregnancy, the more likely they are to have low birth weight, to be born prematurely and to consequently suffer medical problems. For example, in 1999, 53% of babies in multiple births were premature, compared to 7% of singletons. There is also a higher rate of stillbirths for multiples than for singletons.

Virtually all obstetrical risks are increased for the mother during a pregnancy with multiples. As many multiple pregnancies today are the result of the use of fertility therapy, efforts are being made to reduce the risks of multiple pregnancy, specifically triplets or more, by limiting the number of embryos per embryo transfer during IVF to one or two. When fertility drugs are used, many reproductive endocrinologists are also attempting to monitor follicle production via ultrasound. When too many viable eggs are detected, the patient is often advised to cancel the cycle to prevent high-order multiple gestations.

Sometimes a selective reduction — the abortion of one or more of the multiple fetuses — is performed to give the pregnancy a higher chance of producing at least one healthy or live child.

About 75% of women who undergo selective reduction will go into premature labor. About 4-5% of women undergoing selective reduction also miscarry one or more of the remaining fetuses. The risks associated with multi-fetal pregnancy are considered higher.source

Cultural aspects

Certain cultures consider multiple births a portent of either good or evil. ^[2]

In Ancient Rome, the legend of the twin brothers who founded the city (Romulus and Remus) made the birth of identical twin boys a blessing, while identical twin girls were seen as an unlucky burden.

See also

• Biological reproduction
• Conjoined twins
• Dionne Quintuplets
• List of multiple births
• List of twins
• Twin
• Category:Multiple births

References

External links

• HFEA Factsheet on Multiple pregnancies
• Social community and support network for multiples
• Articles on multiple births
• Mothers of Supertwins (MOST) - an international support group for families of triplets, quadruplets, quintuplets, or more offering support, information, resources, and much more.
• Ottawa Canada's Multiple Birth Families Association (MBFA)
• Facts About Multiples: an online encyclopedia of multiple births
• Multiple Births Canada- The web site of the only national organization representing Canada's multiple-birth community providing support and information on multiple-birth topics from conception through school and marriage.
• Multiple Births Foundation- aiding twins families and professionals caring for multiple birth families.
• TAMBA - The web site of the only parent organisation in the UK providing information and support to couples who are expecting, or who have had, a multiple birth.
• National Organization of Mothers of Twins Clubs, Inc (NOMOTC) - A Support Group for Parents of Twins and Higher Order Multiples in the United States.
• MultipleBirth.com - A unique resource for research and information about the causes, effects and problems of multifetal pregnancy.
• What are a woman's chances to have twins?
• Collected stories of full-term multiple pregnancies

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