Objectives. To determine the risk factors for hepatitis B virus infection among pregnant women in Lagos, Nigeria, and the possible implications for hepatitis B prevention in the country. Design. Prospective case control study. Setting. Lagos Island Maternity Hospital. Methods. Between 1 August 2006 and 31 January 2007, risk factors for hepatitis B infection were determined amongst pregnant women using a structured questionnaire. The women were tested for hepatitis B surface antigen (HBsAg) as part of routine antenatal care. Univariate and multivariate analyses were carried out using logistic regression. Main outcome measures. Risk factors for hepatitis B virus infection among pregnant women. Results. Of the 1,052 women attending the antenatal clinic, 6.08% (n = 64) were positive for HBsAg. A total of 61 HBsAg positive and 183 negative controls were interviewed. The significant risk factors for HBV infection were an early age of sexual debut below 19 years (adjusted OR = 2.79; 95% CI = 1.44–5.40; p = 0.0023); history of multiple sexual partners (adjusted OR = 2.02; 95% CI = 1.02–3.98; p = 0.0427); and past history of sexually transmitted infection (adjusted OR = 2.61; 95% CI = 1.15–5.90; p = 0.0214). Of the HBsAg positive women, 45 (73.77%) had at least one of these risk factors while 91 (49.73%) of the controls had at least one risk factor. Conclusion. Screening pregnant for hepatitis B infection on the basis of risk factors may not be effective. Education on modification of lifestyle and sexual behavior as well as non-selective screening of pregnant women for HBV infection is recommended.