Fetal Development

How many different types of twins are there?


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2011-09-20 04:15:47
2011-09-20 04:15:47

Strange as it sounds, there are ten possible types of twins. Three of them are dizygotic (fraternal) and seven of them are monozygotic (identical).

Dizygotic twin types:

  • Fraternal twins, born of two eggs fertilized at the same time. This is by far the most common type of multiple birth.
  • Fraternal half-twins (heteropaternal superfecundation), born of two eggs fertilized at the same time by two different fathers.
  • Superfetation, a rare condition in which another egg is released, and then fertilized, while the mother is already pregnant. This results in two fetuses at different stages in their development, sometimes by a month or more.

Monozygotic twin types:

  • Identical twins, born from a single zygote (fertilized egg) that then splits, usually within the first week of conception.
  • Mirror twins, identical except for having opposite features (including handedness, hair growth, etc.), usually formed from a single zygote that splits sometime after the first week of conception.
  • Conjoined twins, born physically linked to each other, believed to result from a single zygote that splits, then partially reattaches during the course of the pregnancy. The earlier they rejoin, the greater the level of interpenetration. These are often referred to as Siamese twins, after a famous example who lived in 19th-century Thailand (then called Siam).
  • Parasitic twins (fetus in fetu), a very strange form of conjoined twin in which one of the zygotes fails to develop properly, and instead becomes a living parasite inside the body of the surviving twin.
  • Polar body twins, a very rare and not well-understood condition caused by two different sperm somehow fathering the egg after it has already split once; the smaller part of such a split is called the polar body, and is usually discarded as leftover or waste material. Since the resulting twins share the same DNA on the mother's side, they're somewhere between identical and fraternal; which is why until recently they were called semi-identical twins (see below).
  • Aneuploidal twins, resulting from a genetic abnormality in the sex-determining (X and Y) chromosomes; it requires that at least one of the parents have an abnormal number of chromosomes, a condition known as aneuploidy.
  • Semi-identical twins, a unique variation on polar-body twinning in which two of the father's sperm penetrate the same egg (rather than one to the egg, and one to the egg's polar body). This is the rarest known form of twinning, with only one documented case so far.

The last three (polar body, aneuploidal, and semi-identical twins) are the only forms of monozygotic twinning in which it's possible for the siblings to be of opposing gender. And in all three cases, the unusual conditions which cause them virtually guarantee that at least one, if not both, siblings will have developed some form of aneuploidy themselves.


Related Questions

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It depends on what blood types the parents have. If they have the same blood type, most likely the twins will have the same. The opposite with different blood types.

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There are two types of twins -- monozygotic ("identical") from a single egg, and dizygotic (fraternal) from two eggs. Only dizygotic twins can have a different sex.

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Their are two, identical and fraternal.

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Identical twins, fraternal twins and semi- indentical

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No because that is genetic and identical twins carry the same genes

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