Little is known about how pregnancy influences viremia levels in women with chronic hepatitis B virus infection. In this study, we first retrospectively analysed changes in HBV DNA levels during and after 55 pregnancies in HBsAg-positive women, of whom 9 were HBeAg-positive. Secondly, HBV DNA levels in 3 HBeAg-positive mothers whose babies became chronic HBV carriers, were compared with levels in 18 mothers whose babies were not infected by HBV. We found that HBV DNA ranged from 10^8.1 to 10^9.5 copies/mL in HBeAg-positive, and from undetectable (<100) to10^6.8 copies/mL in HBeAg-negative mothers. HBV DNA increased by a mean of 0.4 log late in pregnancy or early post partum; in 4 out of 16 HBeAg negative mothers by >1 log during pregnancy. Post partum ALT increased in both HBeAg-positive and negative women. HBV DNA was 10^9.4–10^10.4 copies/mL in 3 HBeAg-positive mothers whose babies were, as compared to <100–10^10.4 copies/mL in 18 whose babies were not, vertically infected. Although the majority of HBeAg-negative women had low and relatively stable HBV DNA during pregnancy, viremia was also relatively high in some HBeAg-negative mothers, and both viremia and ALT increased significantly late in pregnancy or shortly after delivery. Vertical transmission was only seen in HBeAg-positive mothers with very high levels of viremia. The value of measuring HBV DNA in the pregnant woman to modify immunoprophylaxis to her infant needs further study.