Confirmation of prenatal diagnosis of sex chromosome mosaicism.

Prenatal diagnosis of mosaicism causes problems in interpretation and in genetic counselling. Part of the difficulty with any prenatal diagnosis of mosaicism is interpretation of results without knowing the exact origin, embryonic or extraembryonic, of the abnormal cell line. To confuse the issue in cases of prenatal diagnosis of 45,X/46,XY mosaicism is the recent demonstration that a diagnosis of 45,X/46,XY made prenatally is not necessarily associated with the same phenotype as when diagnosed postnatally. We present two cases of prenatal diagnosis of sex chromosome mosaicism (45,X/46,XY and 45,X/47,XYY). Posttermination examination of the phenotypically normal male fetuses and their placentas established that the placenta was the most likely source of the 45,X cell line. An approach to confirming the prenatal diagnosis of sex chromosome mosaicism and establishing its origin utilizing detailed cytogenetic examination of both fetus and placenta is suggested.