The placenta is an important organ that functions during pregnancy to provide a fetus with oxygen and nutrients and take away waste. It forms along the uterine wall of the mother and connects to the fetus via the umbilical cord.
The number of placentas present in a multiple pregnancy can vary, and can be an indicator of the zygosity of the twins.
Dizygotic or Fraternal Twins
Dizygotic or fraternal twins will always have two placentas. Dizygotic twins form from two separate egg/sperm combinations and each embryo will develop a placenta. Sometimes however, placentas that grow in close proximity may overlap or fuse and can appear to be a single organ when viewed by ultrasound.
Monozygotic or identical twins can have individual or shared placentas, so the number of placentas can vary. Monozygotic multiples form from a single egg/sperm combination that splits after conception. If the split happens within a few days, they will form much like dizygotic twins, implanting separately in the uterus and developing separate placentas.
However, if the split is delayed for a few days, the embryos will develop with a single, shared placenta. In the majority of cases, these multiples will be enclosed within a shared chorion (the outer layer of the sac that contains a fetus) but will develop individually within separate amnions (the inner membrane surrounding the sac of amniotic fluid). The term monochorionic is used to describe this situation. Monochorionic twins may be at risk for Twin-To-Twin Transfusion Syndrome.
Less than five percent of monozygotic twins split a week or more after conception and develop with not only a shared placenta and chorion, but contained within a single amnion. Monoamniotic -- or MoMo -- twins are at risk for cord entanglement and other complications, and the pregnancy must be closely monitored.
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