Background: Preterm birth before 37 weeks' gestation is associated with 70% of perinatal morbidity and nearly half of long-term neurological morbidity. Hospital-based studies have shown that bacterial vaginosis is associated with preterm birth.
Aim: To estimate the relative risk of preterm birth in women with and without bacterial vaginosis, detected by self-administered vaginal swab at < 10 weeks' gestation.
Design: Prospective cohort study.
Setting: Thirty-two general practices and five family planning clinics in South London.
Participants: A total of 1216 women with bacterial vaginosis status established before 10 weeks' gestation, by analysis of Gram stained vaginal smears by two independent observers.
Method: All women who did not miscarry or have a termination of pregnancy before 16 weeks' gestation were sent a brief confidential questionnaire at 16 weeks and at term asking about pregnancy outcome. Data on non-responders were obtained by searches of hospital and general practice records and by telephone calls to patients.
Results: Ascertainment was 87% (937/1072). The mean age of the women was 31 years. Thirteen per cent (122/925) had bacterial vaginosis and 5% (44/897) had a spontaneous preterm birth. The relative risk (RR) of preterm birth in women with bacterial vaginosis was 0.9 (95% confidence interval [CI] = 0.4 to 2.2). However, bacterial vaginosis was associated with late miscarriage at 13-23 weeks (R = 4.0, 95%CI = 1.3 to 12.1). Preterm birth was not associated with previous preterm birth, black ethnicity, age < 20 years, low social class, single marital status, or chlamydial infection. However, it was more common in women who reported smoking in pregnancy (RR = 2.9, 95% CI = 1.5 to 5.5). Of 867 responders, 552 (64%) said that providing a vaginal swab was at least as easy as providing a urine specimen.
Conclusions: In this low-risk community-based cohort, bacterial vaginosis was not a strong risk factor for preterm birth.