What Types Of Identical Twins Are There?


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Julii Brainard answered
Identical twins come in four varieties. When embryos divide very early (by about 3.5 days old) the twins will have separate placentas. This type of pregnancy carries the same risks as a fraternal twin pregnancy.

If the embryos separate between 4 and 9 days old, the twins end up sharing a placenta. These are mono-chorionic twins, the most common type of identical twin (about 67%). There are increased risks of premature birth and disability for this type of twin.

Not only may the placenta may be shared, but in some cases the twins may be inside the same amniotic sac, too (mono-amniotic and mono-chorionic).

Embryos that separate after 12 days end up being conjoined twins (very rare), who naturally also share amniotic sack and placenta.

For all mono-chorionic twins there is a high risk that one twin will grow at the expense of the other. The current advice for mono-amniotic twins is that they should be born by Cesarian section at about 34 weeks. This is a trade off between development and the chance that the umbilical cords will get tangled up.

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