Twin biology: Chorionicity of the placenta

During a twin pregnancy, the main business is to make sure the mother and her babies are well nourished; that the pregnancy goes as near to term as possible; that common complications are anticipated (increased risk of hypertension and hemorrhage); and, that support systems are in place when the twins come home. Chorionicity of the placenta is the most important prenatal issue.

Although it is well known that twin pregnancies carry higher risks than single pregnancies, it is not always realized that some types of twins are more likely to have very high risks; this is the group who have both twins connected to a single (monochorionic or MC) placenta. For MC twins, the risks of serious, life-threatening complications are up to 10 times higher than for those twins who have one placenta each (dichorionic or DC). The distinction between MC and DC twins can be made by ultrasound exam in the first and early second trimester. The dangers in MC twins are caused by the fact that the circulations of the twin pair are usually connected with each other via the placenta. The dangers include: twin-twin transfusion (TTF); twin reversed arterial peffusion (TRAP); unequal sharing of the placenta by the twins, leading to growth discordance; brain damage to one twin if the other should die during fetal life; monoamniotic (MA) twins sharing a single gestational sac with danger of umbilical cord entwinement; conjoined twins. In addition, the rate of major malformation is higher in these twins than in other twins or singletons. Usually, one twin is malformed and the other is not. The malformed twin fetus may threaten the life of the co-twin, and consideration is sometimes given to selective termination; this is highly dangerous in MC twins because of the shared circulations.

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