Monochorionic Diamniotic (MCD) twin pregnancy is a type of twin gestation where two fetuses share a single placenta but develop within separate inner sacs. This arrangement results in genetically identical twins because they originate from a single fertilized egg. The shared placental environment means this pregnancy is considered high-risk and requires specialized, frequent monitoring.
Understanding Chorionicity and Amniosity
The classification of any twin pregnancy depends on two biological factors: chorionicity and amniosity. Chorionicity refers to the number of outer membranes (chorions), which ultimately determine the number of placentas. Sharing this structure defines a monochorionic pregnancy.
Amniosity describes the number of inner membranes (amnions), which form the fluid-filled sacs where the fetuses grow. In a diamniotic pregnancy, each fetus has its own distinct amniotic sac, providing a physical barrier between them. MCD twins are characterized by having one chorion (Mono-) and two amnions (Di-).
Chorionicity dictates the potential risks of the pregnancy. A shared placenta means that blood vessels are connected within the placental tissue, which can lead to unique complications. Separate amniotic sacs prevent complications like umbilical cord entanglement, which is a major concern in monochorionic monoamniotic pregnancies.
How MCD Twins Form
MCD twins arise from a single fertilized egg (zygote) that divides into two embryos. This single-egg origin means MCD twins are always monozygotic, or genetically identical. The timing of the division determines the final configuration of the placenta and sacs.
MCD twins form when the zygote splits between four and eight days after fertilization. During this intermediate timeframe, the outer layer forming the chorion and placenta is established, but the inner cell mass has not fully differentiated. This allows the embryos to share one outer chorionic structure while developing two separate inner amniotic sacs. Splitting before day four results in two separate placentas and sacs, while splitting after day eight leads to the high-risk scenario of sharing both the placenta and a single amniotic sac.
Specialized Monitoring and Diagnosis
The early and accurate diagnosis of chorionicity is important because it dictates the specialized care required throughout the pregnancy. Ultrasound performed in the first trimester, ideally between 11 and 14 weeks of gestation, is the primary diagnostic tool. The sonographer focuses on the junction where the dividing membrane meets the placenta.
The “T-sign” confirms monochorionic status. This visual marker appears as a thin membrane intersecting the placenta at a 90-degree angle, forming a capital ‘T’. This distinct junction is created by the absence of chorionic tissue extending into the membrane. Conversely, a dichorionic pregnancy displays a “Lambda” or “Twin-Peak sign,” where a triangular projection of placental tissue extends into the dividing membrane.
Due to the shared placenta, MCD pregnancies require frequent monitoring. Ultrasound surveillance is initiated early in the second trimester and performed every two weeks. This frequent schedule allows medical providers to promptly detect complications arising from the shared circulation.
Unique Risks Associated with MCD Pregnancies
The single shared placenta in MCD pregnancies is the source of several unique complications. These issues are due to vascular anastomoses, or connections between the blood vessels of the two fetuses, present in over 95% of monochorionic placentas. An imbalanced blood flow through these connections can result in severe health issues for one or both twins.
One complication is Twin-to-Twin Transfusion Syndrome (TTTS), affecting 10 to 15 percent of MCD pregnancies. This condition involves a net transfer of blood from the donor twin to the recipient twin through the shared placental vessels. The donor twin can become severely anemic and dehydrated. Conversely, the recipient twin can become volume-overloaded, leading to heart strain and excess amniotic fluid.
Selective Intrauterine Growth Restriction (sIUGR)
Selective Intrauterine Growth Restriction (sIUGR) occurs when the placenta is unequally divided. One twin receives a significantly smaller share of the placental mass and nutrients, resulting in one twin being much smaller than the other.
Twin Anemia Polycythemia Sequence (TAPS)
Twin Anemia Polycythemia Sequence (TAPS) is a less common complication. It is characterized by a slow, chronic imbalance in red blood cell count through tiny placental connections. This causes one twin to develop severe anemia and the other to have an excess of red blood cells (polycythemia).