Monochorionic/diamniotic twinning refers to what placental arrangement?

• A. One placenta with two amniotic sacs
• B. Two placentas with two amniotic sacs
• C. One placenta with one amniotic sac
• D. Two placentas with one amniotic sac

Answer: (A)

This item tests understanding of how twins are arranged in the uterus based on chorionicity and amnionicity. Monochorionic means the twins share a single placenta, while diamniotic means there are two separate amniotic sacs. When these occur together, the twins have one placental mass but two amniotic sacs. This happens when the zygote splits after the chorion has formed but before the amnion forms, roughly after day 4 but before day 8. On ultrasound you’d see a single placenta with two separate amniotic sacs separated by a dividing membrane that contains two layers of amnion. This arrangement carries a higher risk of shared placental vessels and conditions like twin-twin transfusion syndrome, so monitoring differs from twins with two placentas or with one sac. Therefore, the description is one placenta with two amniotic sacs.