How Many Placentas Are There in a Twin Pregnancy?

Twins may share a placenta or each have their own

Ultrasound Examination In Doctors Office
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When twins or multiples are born, each baby has the same needs for oxygen, nutrients, and waste removal. Does each baby get its own placenta, though? The answer is not simple and depends on a few variables.

The placenta is an important organ. During pregnancy, its function is to provide a fetus with oxygen and nutrients. It also takes away waste. The placenta 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 a single placenta that is shared by the 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 Fraternal Twins

Dizygotic or fraternal twins will always have two placentas. Also called "sibling" twins or "false" twins, the babies are just as unique as any other siblings.

Dizygotic twins form from a combination of two separate eggs and two individual sperm. In this case, each embryo will develop its own placenta.

Sometimes, however, placentas that grow in close proximity may overlap or fuse. This may 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 and this confusion is often a factor.

Placentas Differ for Identical Multiples

Monozygotic or identical twins (also called "real" twins) can have individual or shared placentas, so the number of placentas can vary. Monozygotic multiples form from a single egg and 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). 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.

Triplets and Beyond

In the case of triplets (and other multiples beyond twins), the embryos may form in a combination of ways. Just like twins, they may all share a placenta and be monochorionic.

It's also possible for one embryo to have a separate placenta while the other two share one. This is called dichorionic and the babies that share a placenta may be identical while the other baby is not.

When each of the three babies has their own placenta, the term trichorionic is used. Likewise, when four babies each have their own placenta, it is known as quadchorionic, and so on.

Possible Complications

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, 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 and laser surgery is 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.

This type of pregnancy must be closely monitored. The twins are at risk for cord entanglement, cord compression, and other complications. This is because they each have an umbilical cord but are in the same amniotic sac, allowing them to become intertwined and possibly damaged.

A Word From Verywell

Pregnancy comes with many questions and one involving multiples comes with even more. Knowing how the placenta arrangement of your babies works is rather interesting. Just keep in mind that sometimes the ultrasound can be deceiving.

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Article Sources
  • Benirschke K, Kaufmann P. Pathology of the Human Placenta. Berlin: Springer Science & Business Media, 2013.
  • Minnesota Center for Twin and Family Research. MTFS Twin Info and Frequently Asked Questions. University of Minnesota. 2007.