MALFORMATIONS IN TWINS

Both malformations and twinning are rare events in humans, and epidemi ological studies of their association has required large study populat ions and has provided conflicting results. The overall incidence of ma lformations seems to be increased in single-ovum twins but not in doub le-ovum twins. Abnormalities that may be specific of single-ovum twinn ing include heterokaryotic monozygosity, disorders stemming from vascu lar anastomoses between the twins (fetal transfusion syndrome, anasarc a, porencephaly, visceral necrosis), Siamese twinning, acardiac fetus, in foetu fetus, and adhesions. Ovum removal and prenatal management o f malformations raise greater technical challenges and carry an increa sed risk of complications in twin pregnancies than in singleton pregna ncies. Determination by ultrasonography of single-chorion or double-ch orion status Is essential to the management of malformations in twins.