Overview
The placenta is an important organ that forms along the uterine wall and connects to the fetus via the umbilical cord. Its function is to provide a fetus with oxygen and nutrients during pregnancy, as well as take away waste. With multiples, the number of placentas can vary. There can be more than one placenta (one per fetus) or a single placenta that is shared by the fetuses. 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).
Fraternal Twin Placentas
Dizygotic or fraternal twin pregnancies almost always have two placentas. Also called “sibling” twins or “false” twins, the babies are just as unique as any other siblings. Rarely, there have been cases of monochorionic placentation—in which twins share one placenta but have their own amniotic sac (monochorionic-diamniotic or Mo-Di for short). It’s not clear why it occurs, but the cases were found in people who had used assisted reproductive technology to get pregnant. Dizygotic twins form when two separate eggs combine with two individual sperm. Each embryo develops its own placenta. However, placentas that grow in close proximity sometimes overlap or fuse. When seen on an ultrasound, it might appear to be a single organ.
Identical Twin Placentas
Monozygotic or identical twins (also called “real” twins) can have individual or shared placentas. 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). When this occurs, the term monochorionic-diamniotic (MoDi) is used.
Placentas for Triplets and Beyond
In the case of triplets (and other multiples beyond twins), the embryos can form in several different ways. Just like twins, all the fetuses might share a placenta and be monochorionic. It’s also possible for one embryo to have a separate placenta while the other two share one. With triplets, if 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). With a multiples pregnancy, the number of placentas may determine whether your babies are identical—but sometimes there are other factors to consider.
Possible Complications
Monochorionic twins might be at risk for twin-to-twin transfusion syndrome (TTTS), which occurs in about 10% to 15% of monochorionic twins. With this condition, the blood vessels allow unequal blood flow to each twin. One twin experiences 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 in some cases, laser surgery is performed. 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. This is termed monoamniotic-monochorionic (MoMo) twins and occurs in less than 1% of twin births. This type of pregnancy must be closely monitored because the twins are at risk for cord entanglement, cord compression, and other complications. The twins each have an umbilical cord but are in the same amniotic sac—which means they could become intertwined or develop other cord-related complications, such as reduced blood supply.
A Word From Verywell
Pregnancy comes with many questions—and one involving multiples comes with even more. It’s interesting to learn how the placenta arrangement of your babies works, but it’s also important for ensuring that you have a healthy twin, triplet, or other multiple pregnancy. A varying number of placentas can be deceiving on an ultrasound, but once you know you are carrying multiples, your healthcare provider will keep a close watch on the growth and development of all the fetuses to ensure there are no complications.
