Antenatally detected Klinefelter's syndrome in twins.

An increased incidence of twinning has been reported among patients with Klinefelter's syndrome (XXY) and their relatives, but no data have been reported about the incidence of XXY among twins. Results from our institution's antenatal testing clinic provide data about twins and XXY. In 1842 pregnancies tested, 21 were noted to be twin gestations. Five fetuses with XXY were detected in all pregnancies, of which there was a pair of MZ twins concordant for XXY and a pair of DZ twins discordant for XXY. The incidence of XXY in twins was thus 7,1%. The fetal phenotype of XXY was examined in aborted MZ twins and found to be mild. Notably, the testicular histology was normal. These cases confirm the previously reported association of twinning and XXY. The association of twinning and XXY implies a commonality of causation; however, a maternal age effect cannot be excluded. Our data suggest that there is an increased risk of XXY among twins.