The answer to whether twins always have two placentas is no; the structure of the placenta is directly tied to the type of twins conceived. The placenta is the temporary organ that develops in the uterus during pregnancy, providing oxygen and nutrients to the growing fetus and removing waste products. The number of placentas, or the degree to which they are shared, is determined by how the twins were formed. This biological distinction is important for medical professionals to determine early in a twin pregnancy.
The Genetic Origin of Twins
Twins are broadly categorized into two groups based on their cellular origin. Dizygotic, or fraternal twins, result from two separate fertilization events: two different eggs fertilized by two different sperm cells. Like any other siblings, dizygotic twins share about 50% of their DNA, meaning they can be the same or different sexes.
Monozygotic, or identical twins, develop from a single fertilized egg. One egg is fertilized by one sperm, forming a single zygote that then splits into two embryos early in development. Because they originate from the same fertilized egg, monozygotic twins are genetically nearly identical and are almost always the same sex. The timing of this split determines the structure of the placenta and the surrounding membranes.
Understanding Chorionicity and Amnionicity
The specific arrangement of the placenta and the fetal sacs is described using two terms: chorionicity and amnionicity. Chorionicity refers to the number of chorions, the outermost membranes that contribute to the placenta structure. Amnionicity refers to the number of amnions, the innermost membranes that form the fluid-filled sac surrounding each fetus.
The number of chorions dictates the number of separate placentas or whether a single placenta is shared. There are three main possibilities for twin pregnancies:
- Dichorionic/Diamniotic (Di/Di): Two separate placentas and two separate inner sacs.
- Monochorionic/Diamniotic (Mo/Di): Twins share one chorion and one placenta but are separated by two inner amniotic sacs.
- Monochorionic/Monoamniotic (Mo/Mo): Twins develop within a single placenta and a single amniotic sac.
Determining this arrangement, often in the first trimester, is a crucial step in prenatal care for twin pregnancies.
Placental Structure in Fraternal Twins
Dizygotic, or fraternal, twins are almost always Dichorionic/Diamniotic (Di/Di) because they originate from two separate fertilized eggs that implant independently. Each embryo develops its own chorion, which matures into its own placenta, and its own inner amniotic sac. This arrangement provides the highest degree of separation and independence for each fetus.
Although these twins have two separate placentas, they can sometimes implant close together in the uterine wall. If this occurs, the two placentas may fuse into a single mass, appearing as one on an ultrasound scan. Even when fused, they remain functionally separate, with no significant vascular connections between the two blood supply systems. The placental structure is still classified as Dichorionic.
Placental Structure in Identical Twins
The placental structure in monozygotic, or identical, twins is complex because it depends on the precise moment the single fertilized egg splits. If the split occurs very early, typically within the first three days after fertilization, the twins develop entirely independently. This early split results in a Dichorionic/Diamniotic (Di/Di) pregnancy, meaning each twin has its own placenta and sac, an arrangement seen in about 20% to 30% of identical twin pregnancies.
If the split happens moderately early, between days four and eight post-fertilization, the twins will share a chorion but develop separate amnions. This Monochorionic/Diamniotic (Mo/Di) arrangement accounts for the majority—around 60% to 70%—of identical twin cases. Sharing a single placenta means the twins have vascular connections within the single placental mass, which introduces potential complications.
Finally, a very late split, occurring between nine and twelve days after fertilization, results in the twins sharing both the chorion and the amnion. This Monochorionic/Monoamniotic (Mo/Mo) structure is the rarest form, occurring in only about 1% to 2% of monozygotic pregnancies. The timing of the split dictates the environment, with later division leading to greater sharing of membranes and placental tissue.
Clinical Significance of Placental Type
The determination of chorionicity and amnionicity is a crucial step in managing any twin pregnancy because it directly correlates with the risk of complications. Dichorionic/Diamniotic pregnancies, whether fraternal or identical, carry the lowest risk because the twins have their own separate support systems. The presence of two separate placentas and sacs prevents the sharing of blood supply and provides a physical barrier between the fetuses.
Monochorionic pregnancies, where the twins share a single placenta, have a significantly higher risk profile due to the presence of vascular connections between the twins’ circulatory systems. This shared blood supply can lead to complications such as Twin-to-Twin Transfusion Syndrome (TTTS), where one twin receives too much blood and the other too little. TTTS affects 10% to 15% of monochorionic pregnancies and requires specialized fetal therapy.
The Monochorionic/Monoamniotic (Mo/Mo) type carries the highest risk because the absence of a separating amniotic membrane allows the umbilical cords to tangle. Umbilical cord entanglement is a specific and severe risk for Mo/Mo twins, as fetal movement can restrict the blood flow in one or both cords. The specific placental structure determines the frequency and intensity of prenatal monitoring required.