In this study, HBV-infected pregnant women were divided into two groups, those who received and those who did not receive hepatitis B immunoglobulin (HBIG) during pregnancy. In the mothers, the changes in HBV serum markers (hepatitis B surface antigen (HBsAg), hepatitis B surface antibody (HBsAb), HBeAg, hepatitis B core antibody (HBcAb)), and the DNA load were investigated. Immunohistochemical staining with custom-made antibodies against HBIG revealed both the level and distribution of HBIG in placentas. The protective mechanism of HBIG administrated during pregnancy was explored.