• Fiona M Smaill and Juan C Vazquez.
• Department of Pathology and Molecular Medicine, Faculty of Health Sciences, McMaster University, 1200 Main Street West, Room 2N29, Hamilton, ON, Canada, L8N 3Z5.
• Cochrane Db Syst Rev. 2015 Aug 7 (8): CD000490.

BackgroundAsymptomatic 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.ObjectivesTo assess the effect of antibiotic treatment for asymptomatic bacteriuria on the development of pyelonephritis and the risk of low birthweight and preterm birth.Search MethodsWe searched the Cochrane Pregnancy and Childbirth Group's Trials Register (19 March 2015) and reference lists of retrieved studies.Selection CriteriaRandomized trials comparing antibiotic treatment with placebo or no treatment in pregnant women with asymptomatic bacteriuria found on antenatal screening.Data Collection And AnalysisTwo review authors independently assessed trials for inclusion and risk of bias, extracted data and checked them for accuracy.Main ResultsFourteen 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' ConclusionsWhile 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.

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