Among monozygotic (identical) twins, what percentage are monochorionic diamniotic?

• A. 70%
• B. 30%
• C. 40%
• D. 90%

Answer: (A)

In monozygotic twinning, the placentation type is determined by when the zygote splits after fertilization. If the split happens very early, you get two separate placentas and two amniotic sacs (dichorionic diamniotic). If splitting occurs a bit later, roughly between days 4 and 8, the twins share one chorion but have two amnions (monochorionic diamniotic). Splitting after day 8 can produce a single chorion and a single amnion (monochorionic monoamniotic). The majority of monozygotic twinning falls in that 4–8 day window, so about 70% are monochorionic diamniotic. The remaining cases fall into the other placentation types and are less common.