A triplet pregnancy involves the simultaneous development of three fetuses within the uterus. The placenta is a temporary organ that facilitates the transfer of oxygen and nutrients from the mother’s bloodstream while removing metabolic waste. Understanding its organization is crucial, as the number of placentas, known as chorionicity, directly influences the health and management of the pregnancy. Chorionicity is determined by the earliest biological events of conception and is not always three for three babies.
The Role of Zygosity in Triplet Development
The foundational factor determining the placental configuration is zygosity, which describes how the three fetuses originated from fertilized eggs. Triplets are classified into three primary categories based on the number of eggs and sperm involved. The most common form is trizygotic triplets, resulting from three separate eggs fertilized by three separate sperm, making them fraternal.
Dizygotic triplets occur when two separate eggs are fertilized, and one of those fertilized eggs splits to produce identical twins plus a fraternal sibling. This scenario involves two sperm and two eggs, resulting in a mix of genetic relationships among the babies. The rarest type is monozygotic triplets, originating from a single fertilized egg fertilized by a single sperm that splits twice to create three genetically identical fetuses. The timing of the splits dictates the sharing of the placenta and amniotic sacs.
The likelihood of the fetuses sharing a placenta is directly linked to whether the fertilized egg split and how late in development that split occurred. Monozygotic conceptions inherently involve a higher risk of placental sharing because they begin as a single structure. Trizygotic conceptions, conversely, start as three completely separate entities, which makes placental sharing unlikely. Zygosity thus sets the stage for the crucial decision about the number of placentas, which is called chorionicity.
Mapping Zygosity to Placental Configurations
The number of placentas can range from one to three, and this chorionicity is key to assessing the risks of the pregnancy. Trichorionic-triamniotic (Tri-Tri) is the safest configuration, where each fetus has its own placenta and its own amniotic sac. This arrangement is the typical outcome for trizygotic triplets, although an extremely early split of a monozygotic egg can also result in three separate placentas.
In the trichorionic scenario, the three placentas may implant close enough to appear fused into a single mass at birth. However, they remain biologically distinct with separate fetal circulations. This means there is no vascular connection between the babies, and each fetus receives independent support, which significantly reduces the risks associated with shared vascular systems.
The dichorionic configuration is characterized by two placentas, a common outcome for dizygotic triplets. Two fetuses share one placenta, while the third fetus has its own separate placenta. This is frequently termed dichorionic-triamniotic (Di-Tri) if each baby has its own amniotic sac, or dichorionic-diamniotic (Di-Di) if the two sharing a placenta also share an amniotic sac. The two babies sharing the placenta are almost always monozygotic, or identical, while the third baby is fraternal to the other two.
The rarest and highest-risk configuration is monochorionic (Mo), where all three fetuses share a single placenta. This occurs when a single fertilized egg splits late enough that the three resulting embryos all implant within the same chorion, which develops into the single placenta. Monochorionic triplets are almost exclusively monozygotic, and they are further classified by the number of amniotic sacs, such as monochorionic-triamniotic (Mo-Tri) or the extremely rare monochorionic-monoamniotic (Mo-Mo). Because all three babies rely on a single, shared organ, the risk of complications rises substantially.
Clinical Significance of Placental Sharing
Determining chorionicity is a primary objective during early ultrasound scans because placental sharing introduces medical risks. When two or more fetuses rely on the same placenta, vascular anastomoses (connecting blood vessels) often form between their circulations. These connections can lead to unequal blood flow distribution, which is the root cause of several complications.
One major concern is Selective Fetal Growth Restriction (sFGR), where one or more fetuses receive a disproportionately smaller share of the blood supply and nutrients, leading to significantly slowed growth. Another serious complication is Twin-to-Twin Transfusion Syndrome (TTTS), which can occur in the monochorionic portion of the placenta. In TTTS, blood moves preferentially from a donor fetus to a recipient fetus, causing one baby to become anemic and small while the other becomes overloaded with fluid.
Higher-order multiples, such as triplets, can also develop variations like Twin Anemia-Polycythemia Sequence (TAPS), another form of unequal blood sharing that involves slow, chronic blood transfer. For the rarest monochorionic-monoamniotic triplets, the lack of separate amniotic sacs means the babies’ umbilical cords are free to tangle, posing a risk of cutting off a fetus’s blood supply. For these reasons, pregnancies with shared placentas require intense and frequent prenatal monitoring by specialists to manage these specific, life-threatening conditions.