Antibiotics for asymptomatic bacteriuria in pregnancy

  • Review
  • Intervention


  • F Smaill

Dr Fiona Smaill, Professor, Department of Pathology and Molecular Medicine, Faculty of Health Sciences, McMaster University, Room 2N16, 1200 Main Street West, Hamilton, Ontario, L8N 3Z5, CANADA.



Up to 30% of mothers develop acute pyelonephritis if asymptomatic bacteriuria is untreated. Asymptomatic bacteriuria may have a role in preterm birth or it may be a marker for low socioeconomic status which is associated with low birth weight.


The objective of this review was to assess the effect of antibiotic treatment for asymptomatic bacteriuria on persistent bacteriuria during pregnancy, the risk of preterm delivery, and the development of pyelonephritis.

Search strategy

I searched the Cochrane Pregnancy and Childbirth Group trials register. Date of last search: December 2000.

Selection criteria

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

Data collection and analysis

Trial quality was assessed.

Main results

Fourteen studies were included. Overall the study quality was not strong. Antibiotic treatment compared to placebo or no treatment was effective in clearing asymptomatic bacteriuria (odds ratio 0.07, 95% confidence interval 0.05 to 0.10). The incidence of pyelonephritis was reduced (odds ratio 0.24, 95% confidence interval 0.19 to 0.32). Antibiotic treatment was also associated with a reduction in the incidence of preterm delivery or low birth weight babies (odds ratio 0.60, 95% confidence interval 0.45 to 0.80).

Authors' conclusions

Antibiotic treatment is effective in reducing the risk of pyelonephritis in pregnancy. An apparent reduction in preterm delivery is consistent with current theories about the role of infection in preterm birth, but this association should be interpreted with caution.

Plain language summary

Plain language summary

Antibiotics can reduce the risk of kidney infections in pregnancy.

Asymptomatic bacteriuria is a persistent bacterial growth in the urinary tract that has no specific symptoms. It occurs in 5-10% of all pregnancies. It may contribute to preterm birth (before 37 weeks), low birth weight and kidney infection (pyelonephritis) in the mother. Untreated, infections can cause kidney damage. The review of trials found antibiotics are effective in clearing asymptomatic bacteriuria and preventing symptomatic kidney infection in the mother. The incidence of preterm delivery or low birth weight was also reduced. More research is needed.