Antibiotics for bacterial infection in the urine in pregnancy when there are no symptoms

What is the issue?

Can giving antibiotics in pregnancy to women who have a urinary infection but no symptoms improve the outcomes for women and their babies?

Why is this important?

A bacterial infection of the urine without any of the typical symptoms that are associated with a urinary infection (asymptomatic bacteriuria) occurs in a small number (2% to 10%) of pregnancies. Because of the changes happening in their body, pregnant women are more likely to develop a kidney infection (pyelonephritis) if they have a urinary infection, and the infection may also possibly contribute to having a low birthweight baby or preterm birth (before 37 weeks).

What evidence did we find?

The review of trials on antibiotic treatment for women with no symptoms but bacterial infection in their urine found 14 randomized controlled studies involving almost 2000 women. Antibioitcs were effective in reducing the incidence of kidney infection in the mother (11 studies, 1932 women) and clearing the infection from the urine (four studies, 596 women). The incidence of low birthweight babies (six studies, 1437 babies) and preterm births (two studies, 242 women) seemed also to be reduced. None of the studies adequately assessed any adverse effects of antibiotic treatment for the mother or her baby and often the way the study was done was not well described. The three main outcomes were assessed with GRADE and the evidence to support antibiotic treatment to prevent pyelonephritis, preterm birth and birthweight less than 2500 g was judged to be of low to very low quality.

What does this mean?

Antibiotic treatment can reduce the risk of kidney infections in pregnant women who have a urine infection but show no symptoms of infection. Antibiotics may also reduce the chance a baby will be born too early or have a low birthweight. However, the low quality of the evidence makes it hard to know for certain what the effect of treatment will be and more research is needed.

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.

Read the full abstract...

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 strategy: 

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.