BACKGROUND: No larger population-based study of bacterial vaginosis in pregnancy has previously been available. The objective of this study was to examine risk factors for bacterial vaginosis in pregnancy. DESIGN: From a prospective population-based cohort of 3,596 eligible pregnant women, 2,927 (81.4%) completed the study. METHODS: Women were asked to participate in this study at their first prenatal visit at 17 gestational weeks (range 7 + 3 - 24 + 0). Samples from the genital tract were taken at enrollment. Bacterial vaginosis was determined by Amsel's clinical criteria (3 out of 4: pH > 4.5, homogenous discharge, clue cells, and positive amine test). Data were collected from three questionnaires completed during the second and third trimesters and correlated with the diagnosis of bacterial vaginosis. Crude and adjusted relative risks (reproductive, medical, behavioral, sexual, and sociodemographic factors) were computed. RESULTS: At enrollment, bacterial vaginosis was diagnosed in 13.7% of Danish pregnant women. Significant risk factors for bacterial vaginosis were: daily coitus (adjusted relative risk 2.09 [1.43-3.04]), being single (1.76 [1.21-2.56]), smoking more than 10 cigarettes daily at conception (1.59 [1.29-1.93]), previous genital infection with Chlamydia trachomatis or Neisseria gonorrhoeae (1.39 [1.07-1.79]), and consuming 2 or more drinks per week (1.33 [1.02-1.74]) after control for confounding factors. CONCLUSION: In pregnancy, women who have daily coitus, are single, smokers, with a previous sexually transmitted disease, or with high alcohol consumption in pregnancy are at increased risk for bacterial vaginosis. Information on these risk factors may be important when planning preventive and treatment strategies of bacterial vaginosis in pregnancy.