Antibiotics for treating bacterial vaginosis in pregnancy

Bacteria are normally present in the birth canal and are useful in maintaining the health of the vagina. However, if the numbers of some of the bacteria increase, this is called bacterial vaginosis. For some women, there are no symptoms but for others it may cause an unpleasant discharge and may cause some babies to be born too early. These babies can suffer from problems related to their immaturity both in the weeks following birth such as breathing difficulty, infection and bleeding within the brain as well as problems when growing up such as poor growth, chronic lung disease and delayed development.

The review looked to see whether the use of antibiotics in women with bacterial vaginosis reduced the symptoms for women and reduced the incidence of babies being born too early.  We identified  21 trials, involving 7847 women.  We found that antibiotics given to pregnant women reduced this overgrowth of bacteria, but did not reduce the numbers of babies who were born too early. There were adverse effects sufficient to stop treatment or have the treatment changed when antibiotics were used and this needs further investigation. The effect of screening and treating women with abnormal flora needs to be studied in further trials and the effects of screening and treating proven vaginal infections is the subject of another Cochrane review.

Authors' conclusions: 

Antibiotic treatment can eradicate bacterial vaginosis in pregnancy. The overall risk of PTB was not significantly reduced. This review provides little evidence that screening and treating all pregnant women with bacterial vaginosis will prevent PTB and its consequences. When screening criteria were broadened to include women with abnormal flora there was a 47% reduction in preterm birth, however this is limited to two included studies.

Read the full abstract...

Bacterial vaginosis is an imbalance of the normal vaginal flora with an overgrowth of anaerobic bacteria and a lack of the normal lactobacillary flora. Women may have symptoms of a characteristic vaginal discharge but are often asymptomatic. Bacterial vaginosis during pregnancy has been associated with poor perinatal outcomes and, in particular, preterm birth (PTB). Identification and treatment may reduce the risk of PTB and its consequences.


To assess the effects of antibiotic treatment of bacterial vaginosis in pregnancy.

Search strategy: 

We searched the Cochrane Pregnancy and Childbirth Group's Trials Register (31 May 2012), searched cited references from retrieved articles and reviewed abstracts, letters to the editor and editorials.

Selection criteria: 

Randomised trials comparing antibiotic treatment with placebo or no treatment, or comparing two or more antibiotic regimens in pregnant women with bacterial vaginosis or intermediate vaginal flora whether symptomatic or asymptomatic and detected through screening.

Data collection and analysis: 

Two review authors independently assessed trials for inclusion, trial quality and extracted data. We contacted study authors for additional information.

Main results: 

We included 21 trials of good quality, involving 7847 women diagnosed with bacterial vaginosis or intermediate vaginal flora.

Antibiotic therapy was shown to be effective at eradicating bacterial vaginosis during pregnancy (average risk ratio (RR) 0.42; 95% confidence interval (CI) 0.31 to 0.56; 10 trials, 4403 women; random-effects, T² = 0.19, I² = 91%). Antibiotic treatment also reduced the risk of late miscarriage (RR 0.20; 95% CI 0.05 to 0.76; two trials, 1270 women, fixed-effect, I² = 0%).

Treatment did not reduce the risk of PTB before 37 weeks (average RR 0.88; 95% CI 0.71 to 1.09; 13 trials, 6491 women; random-effects, T² = 0.06, I² = 48%), or the risk of preterm prelabour rupture of membranes (RR 0.74; 95% CI 0.30 to 1.84; two trials, 493 women). It did increase the risk of side-effects sufficient to stop or change treatment (RR 1.66; 95% CI 1.02 to 2.68; four trials, 2323 women, fixed-effect, I² = 0%).

In this updated review, treatment before 20 weeks' gestation did not reduce the risk of PTB less than 37 weeks (average RR 0.85; 95% CI 0.62 to 1.17; five trials, 4088 women; random-effects, T² = 0.06, I² = 49%).

In women with a previous PTB, treatment did not affect the risk of subsequent PTB (average RR 0.78; 95% CI 0.42 to 1.48; three trials, 421 women; random-effects, T² = 0.19, I² = 72%).

In women with abnormal vaginal flora (intermediate flora or bacterial vaginosis), treatment may reduce the risk of PTB before 37 weeks (RR 0.53; 95% CI 0.34 to 0.84; two trials, 894 women).

One small trial of 156 women compared metronidazole and clindamycin, both oral and vaginal, with no significant differences seen for any of the pre-specified primary outcomes. Statistically significant differences were seen for the outcomes of prolongation of gestational age (days) (mean difference (MD) 1.00; 95% CI 0.26 to 1.74) and birthweight (grams) (MD 75.18; 95% CI 25.37 to 124.99) however these represent relatively small differences in the clinical setting.

Oral antibiotics versus vaginal antibiotics did not reduce the risk of PTB (RR 1.09; 95% CI 0.78 to 1.52; two trials, 264 women). Oral antibiotics had some advantage over vaginal antibiotics (whether metronidazole or clindamycin) with respect to admission to neonatal unit (RR 0.63; 95% CI 0.42 to 0.92, one trial, 156 women), prolongation of gestational age (days) (MD 9.00; 95% CI 8.20 to 9.80; one trial, 156 women) and birthweight (grams) (MD 342.13; 95% CI 293.04 to 391.22; one trial, 156 women).

Different frequency of dosing of antibiotics was assessed in one small trial and showed no significant difference for any outcome assessed.