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For a discussion of normal renal and urinary tract function in pregnancy, see Chapter 6, Normal Pregnancy.

Asymptomatic Bacteriuria

Essentials of Diagnosis

  • Urine culture demonstrating the presence of bacteria in the urine in the absence of maternal symptoms of urinary tract infection


Asymptomatic bacteriuria is defined as the presence of actively multiplying bacteria in the urinary tract, excluding the distal urethra, in a patient without any obvious symptoms. The incidence is the same in nonpregnant and pregnant females and averages 2–10%; however, a number of physiologic changes that occur during pregnancy predispose a woman to bacteriuria, including increased glucose concentration in the urine and increased stasis due to the relaxant progesterone effect. Risk factors for developing asymptomatic bacteriuria include low socioeconomic status, parity, age, sexual practice, and medical conditions such as diabetes and sickle cell trait. Escherichia coli is the most common offending organism for asymptomatic bacteriuria (approximately 80% of cases). The Klebsiella-Enterobacter-Serratia group, Staphylococcus aureus,Enterococcus, group B Streptococcus, and Proteus are responsible for the remainder of cases.

The primary concern with asymptomatic bacteriuria in pregnancy is that it is associated with an increased risk of both maternal and fetal complications during pregnancy (see later).


Certain underlying medical disorders may predispose a woman to asymptomatic bacteriuria during pregnancy. Asymptomatic bacteriuria is twice as common in pregnant women with sickle cell trait and 3 times as common in pregnant women with diabetes or with renal transplant as in normal pregnant women. Therefore, we advise that monthly routine urine cultures should be performed in these women when pregnant.

Clinical Findings

The diagnosis of asymptomatic bacteriuria is based on isolation of microorganisms with a colony count >105 organisms per milliliter of urine in a clean-catch specimen in a woman who is experiencing no symptoms of urinary tract infection. When obtaining a clean-catch specimen, the patient should be instructed to clean the vulvar area from front to back to avoid contamination of the urine sample.


The main risk of asymptomatic bacteriuria is the development of pyelonephritis. Due to anatomic dilation in the renal system during pregnancy, there is increased stasis of urine in the pregnant urinary tract and a much higher risk of developing an overt infection from the bacteruria. Due to this risk, many authorities advocate screening all pregnant women for bacteruria, and unlike nonpregnant women, a pregnant woman found to have bacteruria should be treated promptly. If asymptomatic bacteriuria is left untreated in pregnancy, up to 40% of patients will develop symptoms of urinary tract infection (UTI), a significant increase from the almost negligible risk in nonpregnant women. Approximately 25–30% of women will develop acute pyelonephritis. With treatment, the rate is less than 10%. Asymptomatic ...

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