500 pregnant patients with HBeAg-positive and HBV-DNA> 106copies /ml who will do their pregnant check in the First Affiliated Hospital of Xi'an Jiaotong University will be enrolled into the study. There will be five groups to be observed. Four groups are taking tenofovir to prevent intrauterine infection during pregnancy. One group is not taking any anti-HBV virus treatments.The clinical value and effectiveness of tenofovir on blocking HBV intrauterine infection will be evaluated; The HBV-DNA infection status of placenta tissue will be checked by quantitative Polymerase Chain Reaction (PCR) to assess the changes of HBV-DNA of placenta after treating with tenofovir and explore the mechanism of tenofovir blocking HBV intrauterine infection. The safety of Tenofovir will be assessed as well.