Various types of chorionicity and amniosity in monozygotic twi | International Up-to-date Clinical Channel, IUCC

Various types of chorionicity and amniosity in monozygotic twins as a result of when the fertilized egg divides
The degree of separation of the twins in utero depends on if and when they split into two zygotes. Dizygotic twins were always two zygotes. Monozygotic twins split into two zygotes at some time very early in the pregnancy. The timing of this separation determines the chorionicity (the number of placentae) and amniocity (the number of sacs) of the pregnancy.

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