Mo/Di twins, short for Monochorionic-Diamniotic, are identical twins who share a single placenta but develop within two separate amniotic sacs. This configuration is the most common form of monozygotic twinning, present in approximately two-thirds of all identical twin pregnancies. The shared placenta creates a circulatory connection that introduces specific, heightened risks not present when each baby has its own placenta. This shared blood supply means Mo/Di pregnancies require specialized and frequent prenatal monitoring to ensure the health and safety of both babies.
The Biological Difference: How Mo/Di Twins Form
Identical twins, or monozygotic twins, begin when a single fertilized egg splits into two genetically identical embryos. The timing of this initial split determines the twin type and the number of shared membranes and placentas. A very early split, occurring within the first three days post-fertilization, results in two separate placentas and two separate amniotic sacs.
The formation of Mo/Di twins results from the zygote dividing slightly later, typically between four and eight days after fertilization. By this stage, the outer layer of the fertilized egg, which develops into the chorion and the placenta, has already formed as a single unit. Because the split happens after this outer layer is established, the twins share one placenta.
However, the inner layer, which forms the individual amniotic sacs, has not yet fully developed or separated, allowing for two distinct inner membranes. This timing allows for the shared placenta (monochorionic) but separate inner sacs (diamniotic). This configuration provides a physical barrier that prevents the umbilical cords from becoming entangled.
Understanding the Terminology: Chorionicity and Amnionicity
The classification of a twin pregnancy relies on two anatomical factors: chorionicity and amnionicity, which describe the number of outer and inner membranes, respectively. Chorionicity refers to the outer membrane (the chorion), which ultimately forms the placenta. Monochorionic means the fetuses share one chorion and one placenta, while dichorionic means they each have their own.
Amnionicity refers to the amnion, the inner membrane that directly surrounds the fetus and contains the amniotic fluid. Diamniotic indicates two separate inner sacs, and monoamniotic means the twins share a single sac. Mo/Di is the shorthand for Monochorionic-Diamniotic, meaning there is one shared placenta and two separate amniotic sacs.
This classification provides a clear distinction from other twin types, such as Di/Di (Dichorionic-Diamniotic) or Mo/Mo (Monochorionic-Monoamniotic). Determining the chorionicity and amnionicity early in the first trimester is important for guiding prenatal care. The diagnosis is confirmed on ultrasound by the presence of a thin membrane separating the two fetuses and a single placental mass.
Specific Complications Associated with Monochorionic Pregnancies
The single, shared placenta in Mo/Di pregnancies contains vascular connections, or anastomoses, between the two fetal circulations. These inter-twin blood vessel connections are the underlying cause of several specific complications that require close observation. The most recognized of these is Twin-to-Twin Transfusion Syndrome (TTTS), which affects approximately 10% to 15% of Mo/Di pregnancies.
TTTS occurs when there is an unbalanced net flow of blood from one twin (the donor) to the other (the recipient) through the placental connections. The donor twin receives too little blood and can develop low amniotic fluid, while the recipient twin receives too much, leading to excess amniotic fluid and potential heart strain. Selective Fetal Growth Restriction (sFGR) is another condition, occurring in approximately 15% of these pregnancies.
sFGR is caused by an unequal sharing of the placental territory, where one twin receives a disproportionately smaller section. This results in one twin being significantly smaller due to restricted nutrient supply, without the amniotic fluid differences seen in TTTS. Twin Anemia Polycythemia Sequence (TAPS), affecting about 5% of cases, is a milder form of blood imbalance. TAPS involves a slow, chronic exchange of red blood cells, leading to one twin becoming anemic and the other polycythemic.
Specialized Prenatal Monitoring and Delivery Planning
Due to the inherent risks of a shared placenta, Mo/Di twin pregnancies are managed as high-risk and require intensified monitoring. Ultrasound surveillance should begin early, recommended every two weeks starting at about 16 weeks of gestation. These frequent scans assess fetal growth, amniotic fluid levels, and blood flow to detect the earliest signs of TTTS or sFGR.
If the pregnancy remains uncomplicated, delivery is planned earlier than a singleton pregnancy to mitigate the risk of complications late in the third trimester. For uncomplicated Mo/Di twins, the optimal timing for delivery is scheduled between 36 and 37 weeks of gestation. Should complications like sFGR arise, delivery may be planned even earlier, often between 32 and 35 weeks.
The mode of delivery—vaginal or Cesarean section—depends on the position of the first twin and other factors. A twin pregnancy alone does not necessitate a surgical delivery. The increased monitoring and planned early delivery are standard protocols designed to provide timely intervention and maximize positive outcomes for both babies.