Level 2 [mid-level] evidence

Continue Reading

The Infectious Diseases Society of America (IDSA) and the United States Preventive Services Task Force (USPSTF) both strongly recommend screening all pregnant women for asymptomatic bacteriuria with urine culture around 12 to 16 weeks gestation and treating women with positive urine cultures with antibiotics. These recommendations are based on a Cochrane review finding that antibiotic treatment of asymptomatic bacteriuria in pregnant women was associated with a decreased incidence of maternal pyelonephritis as well as a reduced risk of preterm birth and low birthweight.1

The quality of evidence included in the Cochrane review is poor, however. All included trials were published before 1987, with 9 of the 14 trials published between 1960 and 1969 when methods for determining gestational age were less accurate. In addition, most of the trials treated women with positive urine cultures with antibiotics that are no longer used for pregnant women and for durations far exceeding the current IDSA recommendation of 3 to 7 days, and only 1 trial reported treatment-related adverse events. Due to this low quality of evidence, a recent cohort study with an embedded randomized trial was performed to assess the need for screening and treating pregnant women for asymptomatic bacteriuria.2

A total of 4,283 women (mean age, 31 years) with singleton pregnancies at 16 to 22 weeks gestation were screened for asymptomatic bacteriuria at 13 centers in the Netherlands and 248 women (5.8%) had positive urine cultures from a single dipslide. Eighty-five women with asymptomatic bacteriuria were then randomly assigned to receive either 100 mg of nitrofurantoin or placebo twice daily for 5 days. The remaining 163 women with asymptomatic bacteriuria declined trial participation because they did not want to receive antibiotics for an asymptomatic condition, but all the women were followed until 6 weeks postpartum. 

Comparing women with asymptomatic bacteriuria treated with nitrofurantoin to women treated with placebo or refusing treatment (Table), there were no significant differences in the rates of pyelonephritis or delivery at < 34 weeks. Comparing untreated or placebo-treated women to women screening negative for bacteriuria, women with asymptomatic bacteriuria had a significantly higher rate of pyelonephritis but no significant difference in the rate of deliveries at < 34 weeks. Furthermore, while the untreated women had significantly higher rates of symptomatic urinary tract infection (20.2%) and recurrent urinary tract infection (8.7%) treated with antibiotics during pregnancy compared to women with negative screenings (7.9% and 2.6%, respectively), there were no significant differences in other adverse maternal or neonatal outcomes. 

Click to enlarge image 

Although pregnant women with untreated asymptomatic bacteriuria had a slightly higher risk of developing pyelonephritis compared to women without bacteriuria, the absolute risk was small and the subsequent disease course was mild. The increase in pyelonephritis as well as that of uncomplicated urinary tract infections did not influence the development of other adverse maternal or neonatal outcomes. However, this study was not large enough to detect differences in most of these outcomes due to low event rates. These results suggest that screening for asymptomatic bacteriuria may not clearly lead to better pregnancy-related outcomes and exposes a large number of women to unnecessary antibiotics. 

Overall, these findings call into question recommendations for routine screening of all pregnant women and for treatment of asymptomatic pregnant women, particularly in the climate of growing antimicrobial resistance. Of note, the European Association of Urology has recently updated its guidelines to make no recommendation for screening or treating pregnant women with asymptomatic bacteriuria based on the lack of benefit derived in this study combined with the low quality of the evidence for benefit in previous studies.3 

Alan Ehrlich, MD, is a deputy editor for DynaMed, Ipswich, Mass., and assistant clinical professor in Family Medicine, University of Massachusetts Medical School in Worcester. 

DynaMed is a database that provides evidence-based information on more than 3,000 clinical topics and is updated daily through systematic surveillance covering more than 500 journals. 


  1. Smaill FM, Vazquez JC. Antibiotics for asymptomatic bacteriuria in pregnancy. Cochrane Database Syst Rev. 2015;[8]:CD000490

  2. Kazemier BM, Koningstein FN, Schneeberger C, et al. Maternal and neonatal consequences of treated and untreated asymptomatic bacteriuria in pregnancy: A prospective cohort study with an embedded randomised controlled trial. Lancet Infect Dis. 2015;15[11]:1324-1333

  3. Pickard R, Bartoletti R, Bjerklund-Johansen TE, et al. 2016 Urological Infections Guidelines. European Association of Urology website. http://uroweb.org/guideline/urological-infections. Accessed September 7, 2016