How Many Placentas Are There in a Twin Pregnancy?

Twins May Share a Placenta or Each Have Their Own

Doctor performing ultrasound on pregnant woman
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When twins or multiples are born, each baby has the same needs for oxygen, nutrients, and waste removal—but does each baby get its own placenta?

The placenta is an important organ. During pregnancy, its function is 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.

With multiples, the number of placentas can vary.

There can be multiple placentas, one per baby, or there can be a single placenta that is shared by both babies. The number of placentas can be an indicator of the zygosity of the twins, a term that refers to whether they developed from the same egg or from different eggs.

Two Placentas for 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.  In fact, the Minnesota Center for Twin & Family Research says fraternal and identical twins are frequently misidentified, with this confusion being a factor.

Monozygotic or Identical Multiples Differ in Placenta Number

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 right away, within a few days post-conception, 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 most will develop individually within separate amnions (the inner membrane surrounding the sac of amniotic fluid). The term monochorionic-diamniotic (MoDi) is used to describe this situation.

Monochorionic twins may be at risk for twin-to-twin transfusion syndrome (TTTS). This happens to about 20 percent of monochorionic twins. In this condition, the blood vessels allow unequal blood flow to each twin. One twin has decreased blood flow and slower growth and less amniotic fluid. The other twin has excess blood flow and too much amniotic fluid, which can result in heart strain. This condition can be managed, with laser surgery being performed in some cases.

Rarely, 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-monochorionic (MoMo) twins occur in less than 1 percent of twin births. They are at risk for cord entanglement, cord compression, and other complications, and the pregnancy must be closely monitored. This is because they each have an umbilical cord but are in the same amniotic sac, allowing them to become intertwined and possibly damaged.

Source:

Benirschke K, Kaufmann P. Pathology of the Human Placenta. Berlin: Springer Science & Business Media, 2013. Print.

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