Antibiotics for asymptomatic bacteriuria in pregnancy

RHL Summary

8 March 2016
A pregnant woman attends a prenatal clinic where a nurse/educator asks her questions, Haiti.
Photoshare/Paul Jeffrey

Key findings

Antibiotic treatment for asymptomatic bacteriuria in pregnant women compared to placebo or no treatment was associated with:

  • Reduction in the incidence of pyelonephritis
  • Reduction in the incidence of low birthweight (<2500g) babies
  • Reduction in preterm births (<37 weeks) (based on two studies)
  • Reduction in persistent bacteriuria at time of delivery

While results are consistent, there was considerable heterogeneity and methodological limitations across all studies.

Evidence included in this review

Fourteen randomized controlled trials and quasi-randomized trials, involving almost 2000 women were included in the review.

Quality assessment

The overall quality of evidence was graded as low to very low because of high or unclear risk of bias. There was significant heterogeneity among trials in terms of study design, quality of studies and populations studied.

Clinical implications

Antibiotic treatment of asymptomatic bacteriuria during pregnancy appears to reduce the risk of pyelonephritis, however the evidence is of low quality.

Further research

Further good quality research is needed to improve certainty about the effect of treatment on the course of infection and the outcome of pregnancy, as well as the appropriate method, timing and frequency of urine screening.

Cochrane review

Citation: Smaill FM, Vazquez JC. Antibiotics for asymptomatic bacteriuria in pregnancy. Cochrane Database of Systematic Review 2015, Issue 8. Art. No.: CD000490. DOI: 10.1002/14651858.CD000490.pub3.



Asymptomatic bacteriuria occurs in 2% to 10% of pregnancies and, if not treated, up to 30% of mothers will develop acute pyelonephritis. Asymptomatic bacteriuria has been associated with low birthweight and preterm birth.


To assess the effect of antibiotic treatment for asymptomatic bacteriuria on the development of pyelonephritis and the risk of low birthweight and preterm birth.

Search methods

We searched the Cochrane Pregnancy and Childbirth Group's Trials Register (19 March 2015) and reference lists of retrieved studies.

Selection criteria

Randomized trials comparing antibiotic treatment with placebo or no treatment in pregnant women with asymptomatic bacteriuria found on antenatal screening.

Data collection and analysis

Two review authors independently assessed trials for inclusion and risk of bias, extracted data and checked them for accuracy.

Main results

Fourteen studies, involving almost 2000 women, were included. Antibiotic treatment compared with placebo or no treatment reduced the incidence of pyelonephritis (average risk ratio (RR) 0.23, 95% confidence interval (CI) 0.13 to 0.41; 11 studies, 1932 women; very low quality evidence). Antibiotic treatment was also associated with a reduction in the incidence of low birthweight babies (average RR 0.64, 95% CI 0.45 to 0.93; six studies, 1437 babies; low quality evidence) and preterm birth (RR 0.27, 95% CI 0.11 to 0.62; two studies, 242 women; low quality evidence). A reduction in persistent bacteriuria at the time of delivery was seen (average RR 0.30, 95% CI 0.18 to 0.53; four studies; 596 women). There were very limited data on which to estimate the effect of antibiotics on other infant outcomes and maternal adverse effects were rarely described.

Overall, all 14 studies were assessed as being at high or unclear risk of bias. While many studies lacked an adequate description of methods and the risk of bias could only be assessed as unclear, in almost all studies there was at least one domain where the risk of bias was judged as high. The three primary outcomes were assessed with GRADE software and given a quality rating. Evidence for pyelonephritis, preterm birth and birthweight less than 2500 g was assessed as of low or very low quality.

Authors' conclusions

While antibiotic treatment is effective in reducing the risk of pyelonephritis in pregnancy, the estimate of the effect is very uncertain because of the very low quality of the evidence. The reduction in low birthweight and preterm birth with antibiotic treatment is consistent with theories about the role of infection in adverse pregnancy outcomes, but this association should be interpreted with caution given the very poor quality of the included studies.

This RHL summary should be cited as: WHO Reproductive Health Library: Antibiotics for asymptomatic bacteriuria in pregnancy: RHL summary (last revised 8 March 2016). The WHO Reproductive Health Library; Geneva: World Health Organization.