The presence of a monozygotic twin gestation with discordant sex of the twins is a very rare constellation, which is referred to as heterokaryotypic monozygotic pregnancy. This constellation can develop either due to a chromosomal 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 pregnancy has always been made postnatally, with only one exception (Gonsoulin et al., 1990). In this case we suspected the presence of monozygotic twins ultrasonically because of the chorionic and amniotic membrane characteristics. Surprisingly the sex of the fetuses was discrepant. As one of them had hydrops and a structural heart defect, we carried out an amniocentesis, which revealed mosaicism [45,X/46,X,i(Y)(p10)] of both fetuses. The female fetus with a predominant 45,X set of chromosomes and the typical intrauterine signs of the Ullrich-Turner syndrome (massive hygroma colli, hydrops fetalis and multiple cardiac defects) died during the 25th week of gestation due to cardiac decompensation. The other fetus appeared to be male with a predominance of a 46,X,i(Y)(p10) set of chromosomes and was born a few days after the intrauterine death of the hydropic fetus. In conclusion, our observation shows that ultrasonic evidence of discordant fetal sex in twins does not necessarily exclude monozygosity.
Copyright 2000 John Wiley & Sons, Ltd.