摘要AIM: To explore the possible mechanism of intrauterine infection of hepatitis B virus (HBV).METHODS: HBV DNA was detected in vaginal secretion and amniotic fluid from 59 HBsAg-positive mothers and in venous blood of their newborns by PCR. HBsAg and HBcAg in placenta were determined by ABC immunohistochemistry.RESULTS: The rate of HBV intrauterine infection was 40.l% (24/59). HBV DNA was detected in 47.5% of amniotic fluid samples and 52.5% of vaginal secretion samples respectively.HBsAg and HBcAg were detected in placentas from HBsAgpositive mothers. The concentration of the two antigens decreased from the mother's side to the fetus's side, in the following order: maternal decidual cells ＞ trophoblastic cells ＞ villous mesenchymal cells ＞ villous capillary endothelial cells. However, in 4 placentas the distribution was in the reverse order. HBsAg and HBcAg were detected in amniotic epithelial cells from 32 mothers.CONCLUSION: The main route of HBV transmission from mother to fetus is transplacental, from the mother side of placenta to the fetus side. However, HBV intrauterine infection may take place through other routes.