Maternal serum hepatitis B virus (HBV) DNA level is considered as the most important and independent risk factor to predict the HBV vertical transmission rates. After fully considered the pros and cons of the mother and fetus, it′s generally agreed that administration of oral nucleoside antiviral in the second and third trimester can significantly reduce maternal serum HBV loads before delivery. Consequently, it will prevent maternal-fetal transmission of HBV to the maximum extent. HBV DNA level at week 4 of antiviral treatment predicts the long-term efficacy, which conduce to improve the therapeutic schedule. Overall, the measurement of HBV DNA level during pregnancy and the optimal time have important meaning for preventing maternal-fetal transmission of HBV.