Mother-to-child transmission (MTCT) is still the main transmission route of HBV in high-endemic areas, such as China, sub-Saharan Africa, etc. Some infants born of mothers with high HBV DNA load (≥2×10\^5 IU/ml) are still infected with HBV even if these infants receive the combined immunization on time. Therefore, guidelines including AASLD and EASL recommend that pregnant women with high HBV DNA load should take antiviral drugs (tenofovir disoproxil fumarate or telbivudine) to reduce MTCT of HBV from gestation 24-28 weeks. However, side effects of TDF on infants are reported. For example, neutropenia and the decrease of bone mineral density are found in early age infants who are ever exposed to TDF during their fetal life. Tenofovir alafenamide (TAF), a new prodrug of tenofovir (TFV), has a higher antiviral potency, a higher peripheral blood mononuclear cell (PBMC) intracellular tenofovir diphosphate (TFV pp) level and a lower plasma TFV concentration. As the successor of TDF, the dose of TAF that is took orally every day is approximately 1/10 of TDF. TAF has a much lower risk of kidney toxicity and has almost no effect on the bone mineral density. TAF has been approved and recommended as the first-line drug to treat patients with chronic hepatitis B (CHB) by AASLD, EASL, etc. However, there are relatively few data of TAF on pregnancies with high HBV DNA load. It is urgently to clarify the safety and efficacy of TAF on interrupting MTCT of HBV in pregnancies with high HBV DNA load. In the present study, the investigators enroll middle/late pregnancies with high HBV DNA load(≥2×10\^5 IU/ml). The participants are randomly divided into two groups. Then the participants are treated with TAF or TDF respectively. All enrolled participants are followed-up for 2 years. Objectives of the present study are as follows: A. To clarify safety and efficacy of TAF on interrupting MTCT of HBV in middle/late pregnancies with high HBV DNA load. B. To clarify effects of TAF on obstetric complications in middle/late pregnancies with CHB. C. To clarify effects of TAF on birth defects of infants born in mothers with CHB. D. To clarify the change of virology and biochemistry indexes in women with CHB during pregnancy and postpartum. E. To clarify effects of TAF treatment on participants. F. To clarify growth parameters of the infants exposed to TAF during their fetal life. G. To clarify the pharmacokinetics of TAF in pregnant populations.