'Identical' twins with discordant karyotypes

A chromosomal abnormality in one of the fetuses of a monozygotic twin pregn ancy is a rare phenomenon. In the prenatal unit of our cytogenetics laborat ory we have recently come across two such heterokaryotypic twin pregnancies . In both cases ultrasound abnormalities were detected in one fetus of each twin pair. Chromosomal analysis showed that one twin pregnancy was discord ant for trisomy 21 and the other for 45,X. Ultrasonographic examination sug gested a monochorionic twin pregnancy in each case and DNA studies confirme d that both sets of twins were monozygotic. Both pregnancies were terminate d. Biopsies taken from different sites of the placentas showed chromosomal mosaicism in both crises. There was no clear correlation between the karyot ype found close to the site of the umbilical cord insertion in the placenta and the karyotype of the fetus. Sampling of amniotic fluid from both sacs is recommended in diamniotic twin pregnancies if one (or both) of the fetus es has ultrasound abnormalities, even if the twins are apparently monochori onic. Copyright (C) 1999 John Wiley & Sons, Ltd.