A monochorionic twin pregnancy is a type of monozygotic twin pregnancy where the twins share a single chorion. Depending on the sharing of the amnion this can be further divided into two types:
monochorionic monoamniotic (MCMA) pregnancy: single amnion
monochorionic diamniotic (MCDA) pregnancy: two amnions (much more common ~90%)
As they are almost always monozygotic (except in extremely unusual cases), both the twins in a monochorionic twin pregnancy are of the same sex.
Pathological processes that occur almost exclusively in monochorionic pregnancies include:
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