Impact of fetal sex in pregnancy-induced hypertension and preeclampsia in Japan.

The male antigen (HY), the elevated level of fetal antigen in twin pregnancies, and the increased number of MHC mismatches in dizygotic twin pregnancies might affect immunological tolerance during pregnancy. Using the Perinatal Database of the Japanese Society for Obstetrics and Gynecology, we studied the occurrence of pregnancy-induced hypertension (PIH) and preeclampsia in mothers delivering singleton babies and in those delivering monochorionic diamniotic (MD) twin pregnancies and dichorionic diamniotic (DD) twin pregnancies at 125 centers of the perinatal network in Japan from 2001 through 2005. In singleton pregnancies, pregnant women carrying female fetuses had a significantly higher incidence of PIH and preeclampsia compared with those carrying male fetuses. In MD twin pregnancies, compared with mothers carrying male-male fetuses, those carrying female-female fetuses had significantly higher incidences of PIH and preeclampsia and a marked difference was observed in primiparous cases. In DD twin pregnancies, the incidences of PIH and preeclampsia were significantly higher in mothers with female-female fetuses than those with male-male fetuses, while those with male-female fetuses had intermediate values. The incidence of PIH and preeclampsia in MD twin pregnancies was similar to that in DD twin pregnancies with male-male fetuses or female-female fetuses. The male antigen and the increased number of MHC mismatches in DD twin pregnancies were not a risk factor for PIH and preeclampsia. Female fetal sex was a risk factor for PIH and preeclampsia.