Identical twin pregnancies are a fascinating aspect of human biology, originating from a single fertilized egg. During gestation, the placenta and umbilical cord play fundamental roles in providing nourishment and oxygen to the developing fetuses. The unique formation of identical twins leads to various arrangements of these vital structures, influencing the course of the pregnancy. Understanding these configurations is important for comprehending the distinct characteristics and potential complexities of identical twin development.
How Identical Twins Form
Identical, or monozygotic, twins develop from a single fertilized egg that splits into two embryos. The timing of this division is a key factor in determining how the twins will share their gestational structures, specifically the placenta, chorion, and amnion. The chorion is the outermost membrane that contributes to the placenta, while the amnion is the inner membrane forming the fluid-filled sac around each fetus.
If the single fertilized egg divides very early, typically within the first three days after fertilization, each twin will develop its own placenta, chorion, and amniotic sac. This early separation results in a pregnancy type that closely resembles fraternal twins in terms of membrane structures. A later split, occurring between four and eight days after fertilization, leads to twins who share a single placenta and chorion but develop in separate amniotic sacs. If the division happens even later, between eight and twelve days post-fertilization, the twins will share both a placenta and a single amniotic sac, making this the rarest form of identical twinning.
Placental and Cord Connections
Identical twins do not typically share the same umbilical cord; instead, each twin develops its own distinct umbilical cord. However, they can share a placenta, depending on when the original fertilized egg divides. This sharing of the placenta is a defining characteristic for many identical twin pregnancies.
There are three types of identical twin pregnancies based on how they share these structures. Dichorionic-Diamniotic (DCDA) identical twins result from the earliest split, where each twin has its own placenta, chorion, and amniotic sac. About 20-30% of identical twins are DCDA, and their placental arrangement is similar to that of fraternal twins.
The most common type of identical twins are Monochorionic-Diamniotic (MCDA) twins, accounting for 70-75% of identical pregnancies. These twins share a single placenta and chorion but develop within their own separate amniotic sacs. While they share a placenta, each twin has its own umbilical cord that connects to this shared placental tissue.
A less common, higher-risk type is Monochorionic-Monoamniotic (MCMA) twins, occurring in 1-5% of identical twin pregnancies. These twins share both a single placenta and a single amniotic sac. Despite sharing the same sac, each MCMA twin still possesses its own umbilical cord. The absence of a dividing membrane means their individual umbilical cords are within the same confined space, increasing the possibility of entanglement.
Risks When Structures Are Shared
When identical twins share a placenta, as in monochorionic pregnancies (MCDA and MCMA), complications can arise due to the shared blood supply. One risk is Twin-to-Twin Transfusion Syndrome (TTTS), which occurs in 10-15% of monochorionic twins. In TTTS, unequal blood vessel connections within the shared placenta lead to imbalanced blood flow between the twins: one receives too much blood (recipient) and the other too little (donor).
Another complication is Selective Fetal Growth Restriction (sFGR). This condition is characterized by one twin growing slower than the other, often due to unequal sharing of the placenta, where one twin receives a smaller portion of the placental resources.
For MCMA twins, the absence of a dividing membrane within the shared amniotic sac presents a challenge: umbilical cord entanglement. As the twins move within the single sac, their individual cords can become intertwined, potentially restricting blood flow and nutrient supply to one or both fetuses. While some cord entanglement is common in nearly all monoamniotic pregnancies, severe entanglement can lead to serious complications.