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Describe the four types of monozygotic twinning and when division occurs to produce each type of twin.
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Correct answer:
The most common type of monozygotic twinning is diamniotic monochorionic twins. This results from division 3–8 days after fertilization and produces a pregnancy with two amniotic sacs and a single placenta. The second most common type is diamniotic dichorionic twins. This results from division of the embryo within the first 72 hours after fertilization and produces a pregnancy with two amniotic sacs and two placentas. Another type of monozygotic twining is monoamniotic monochorionic, which is rare and is a result of division after the amnion forms on day 8 post-fertilization but before day 12. Lastly, conjoined twins are the rarest form of monozygotic twinning and arise from cleavage of the differentiating embryonic disk 13–16 days post-conception.
