Prenatal diagnosis of heterokaryotypic mosaic twins discordant for fetal sex

The presence of a monozygotic twin gestation with discordant sex of the twi ns is a very rare constellation, which is referred to as heterokaryotypic m onozygotic pregnancy. This constellation can develop either due to a chromo somal aberration after twinning or is - as in the following case - due to a mitotic error before twinning and an unequal distribution of mosaicism in both embryos. So far the diagnosis of heterokaryotypic monozygotic pregnanc y has always been made postnatally, with only one exception (Gonsoulin ct a l., 1990). In this case we suspected the presence of monozygotic twins ultr asonically because of the chorionic and amniotic membrane characteristics. Surprisingly the sex of the fetuses was discrepant. As one of them had hydr ops and a structural heart defect, we carried out an amniocentesis, which r evealed mosaicism [45,X/46,X,i(Y)(p10)] of both fetuses. The female fetus w ith a predominant 45,X set of chromosomes and the typical intrauterine sign s of the Ullrich-Turner syndrome (massive hygroma colli, hydrops fetalis an d multiple cardiac defects) died during the 25th week of gestation due to c ardiac decompensation. The other fetus appeared to be male with a predomina nce of a 46,X,i(Y)(p10) set of chromosomes and was born a few days after th e intrauterine death of the hydropic fetus. In conclusion, our observation shows that ultrasonic evidence of discordant fetal sex in twins does not ne cessarily exclude monozygosity. Copyright (C) 2000 John Wiley & Sons, Ltd.