Which timing of zygote split yields monochorionic/diamniotic twins?

• A. 3-5 days
• B. 5-10 days
• C. 10-14 days
• D. 0-3 days

Answer: (B)

Monochorionic/diamniotic twins occur when the zygote splits after the chorion has formed but before the amnionic sacs fully separate, so there is one shared placenta but two amniotic sacs. This timing usually falls in the middle of development, around the 5–10 day window after fertilization. Because the chorion forms early to create a single placental boundary, splitting within this period yields one chorion with two amniotic sacs, giving monochorionic/diamniotic twins. If splitting happens earlier, you’d get two chorions (dichorionic/diamniotic); if it happens later (after about day 8–12), you’re more likely to get one chorion with a single amnion (monochorionic/monoamniotic). Hence, the 5–10 day range best matches monochorionic/diamniotic twins.