Monochorionic-Diamniotic Twins (MCDA)

Definition - What does Monochorionic-Diamniotic Twins (MCDA) mean?

Monochorionic-diamniotic 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. In addition to the normal risks associated with twin pregnancies, there are also other complications that can arise because of the shared placenta, such as twin-to-twin transfusion syndrome, fetal growth restriction, and twin reversed arterial perfusion. Monochorionic-diamniotic twins occur in 0.3% of all pregnancies and 75% of all monozygotic twin pregnancies and occur completely randomly.

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 one placenta, but early enough that amniotic sacs aren’t present, so each twin will develop its own. Monochorionic-diamniotic 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 placenta or each has their own. Identifying a monochorionic pregnancy early in the first trimester is important in monitoring any complications that might arise.

As with any type of twin pregnancy, there are increased risks of gestational diabetes, early delivery, and preeclampsia. Monochorionic-diamniotic twins are often delivered between 34 and 36 weeks of pregnancy, because of the increased risk of complications and stillbirth late in the third trimester. Because they share one placenta, monochorionic-diamniotic twins have circulatory systems that are connected with irregular blood vessels. This can cause complications such as twin-to-twin transfusion syndrome (TTTS), which occurs in about 1 of every 5 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 20% 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 (TRAP) can develop, where one twin’s circulatory system doesn’t grow properly, and that twin relies on the other’s heart to pump blood for it. This can put a great amount of stress on the twin doing the pumping. Selective intrauterine grown restriction (sIUGR) 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 trimester of pregnancy, the survival rate of monochorionic twins is nearly 90%.

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