What Does Monochorionic-Diamniotic Twins (MCDA) Mean?
Monochorionic-diamniotic (MCDA) twins are twins that share a placenta but have their own amniotic sac. They are almost always monozygotic, meaning that they come from one egg that is fertilized by one sperm, and therefore are identical.
Monochorionic-diamniotic twins occur in 0.3% of all pregnancies and 70 to 75% of all monozygotic twin pregnancies and occur completely at random.
FertilitySmarts Explains Monochorionic-Diamniotic Twins (MCDA)
After the egg is fertilized by the sperm, the resulting zygote splits into two, at some point between 3 and 8 days after fertilization. This is late enough for the developing embryo to have created the foundation for a shared placenta, but early enough for the amniotic sac to develop, so each twin will develop the sac of its own.
MCDA twins are almost always created from one egg and one sperm, and so are genetically identical, but there are rare cases where two embryos have fused together to create one placenta and two amniotic sacs.
An ultrasound between 10 and 14 weeks of pregnancy allows medical staff to tell if the twins share a common placenta or they have their own. Identifying a monochorionic pregnancy early in the first trimester is important in monitoring any complications that might arise.
Complications with MCDA Twins
As with any type of twin pregnancy, women with MCDA twins are at increased risk of complications like diabetes, early delivery MCDA twins are often delivered between 34 and 37 weeks of pregnancy), and preeclampsia.
Because they share one placenta, MCDAtwins have circulatory systems that are connected with irregular blood vessels. This can cause complications, in particular, twin-to-twin transfusion syndrome (TTTS), which occurs in about 10% of monochorionic-diamniotic pregnancies. TTTS causes blood to be diverted unevenly to one twin over the other, meaning that one twin gets too much blood which the other gets too little. The smaller twin ends up being about 25% smaller in weight than its counterpart, which could be too small to grow normally or even survive.
Even without the development of TTTS, there can still be a distinct difference between the twins in terms of birth weight. In rare cases, a condition called twin reversed arterial perfusion can develop, where one twin’s circulatory system doesn’t grow properly, and that twin relies on the other’s heart to pump the blood for it. This can put a great amount of stress on the twin doing the pumping.
Selective intrauterine growth restriction is a condition characterized by an uneven sharing of the placenta, which can lead to unusual blood flow to the fetuses, and ultimately slow growth or no growth for one twin.
These conditions are rare, and, if detected in the first three months of pregnancy, the survival rate of monochorionic twins is nearly 90%.