What We Know: Asymptomatic bacteriuria in pregnancy

January 1, 2009

Here's practical, evidence based advice on what to tell a pregnant with asymptomatic bacteriuria.

This department will take a hard look at some of the common things done in obstetrics/gynecology practice, such as supplementing iron during prenatal care, giving antibiotics prior to gynecologic surgery, and screening for asymptomatic bacteriuria, and will review the evidence supporting the approach. By examining the available evidence, using resources such as randomized clinical trials, meta-analyses, and the Cochrane Collaboration, we will make a blunt assessment of what we know, and how effective our interventions really are. In many cases, the results may be surprising, and in some cases, this department will be less about 'What We Know,' and more about 'What We Think We Know But Really Don't.'

CASE: A 32-year-old G1 P0 at 12 weeks' gestation comes to your office for her second prenatal visit. Since you saw her last, she recently completed the prenatal screening tests that you had ordered, and you note that she has more than 100,000 units mL Escherichia coli reported in her urine culture. As you hand her a prescription for an antibiotic, she asks, "I am nervous about taking these pills while I am pregnant; how important is it for me to fill the prescription? I've read a lot about eating healthy in pregnancy and know it is important to avoid unpasteurized foods, avoid excessive mercury in some fish, and to not take any medications."

Asymptomatic bacteriuria (AB) refers to the presence of a positive urine culture in a patient who has no symptoms of infection. AB occurs in about 5% of pregnant women, and E coli and Klebsiella are among the most common organisms identified.1,2 Due to physiologic changes in pregnancy, such as the mass effect of the enlarging uterus and smooth muscle relaxation secondary to a change in the hormonal milieu, pregnant women seem especially prone to AB.3 AB is typically diagnosed following one to two clean-catch urine cultures that reveal more than 100,000 colony-forming units per milliliter of the same organism.4

AB is also associated with adverse pregnancy outcomes. Low birthweight, perinatal mortality, and preterm birth have all been firmly linked to AB,9 although studies have been of variable quality.5 Nevertheless, antibiotics were associated with a reduction in preterm deliveries or low birthweight babies (OR, 0.60; 95% CI, 0.45–0.80).8

Having reviewed the research, you can assure your patient with confidence that it is vitally important that she fill the prescription. The evidence shows that screening and treatment of AB is one of the simplest and most important components of prenatal care.


1. Gilstrap LG 3rd, Hankins GD, Snyder RR, et al. Acute pyelonephritis in pregnancy. Compr Ther. 1986;12:38-42.

2. Leveno KJ, Harris RE, Gilstrap LC, et al. Bladder versus renal bacteriuria during pregnancy: recurrence after treatment. Am J Obstet Gynecol. 1981;139:403-406.

3. Colgan R, Nicolle LE, McGlone A, et al. Asymptomatic bacteriuria in adults. Am Fam Physician. 2006;74:985-990.

4. Nicolle LE. Asymptomatic bacteriuria: review and discussion of the IDSA guidelines. Int J Antimicrob Agents. 2006;28(suppl 1):S42-S48.

5. Smaill F. Asymptomatic bacteriuria in pregnancy. Best Pract Res Clin Obstet Gynaecol. 2007;21:439-450.

6. Elder HA, Santamarina BA, Smith S, et al. The natural history of asymptomatic bacteriuria during pregnancy: the effect of tetracycline on the clinical course and the outcome of pregnancy. Am J Obstet Gynecol. 1971;111:441-462.

7. Kass EH. Bacteriuria and the pathogenesis of pyelonephritis. Lab Invest. 1960;9:110-116.

8. Smaill F. Antibiotics for asymptomatic bacteriuria in pregnancy. Cochrane Database Syst Rev. 2001;(2):CD000490.

9. Naeye RL. Causes of the excessive rates of perinatal mortality and prematurity in pregnancies complicated by maternal urinary-tract infections. N Engl J Med. 1979;300:819-823.