Williams Manual of Pregnancy Complications, 23 ed.

CHAPTER 63. Asymptomatic Bacteriuria

Asymptomatic bacteriuria (ASB) refers to persistent, actively multiplying bacteria in women who have no symptoms. The incidence during pregnancy varies from 2 to 7 percent and depends on parity, race, and socioeconomic status. The highest incidence is in African American multiparas with sickle-cell trait, and the lowest incidence is in affluent white women of low parity. Bacteriuria is typically present at the time of the first prenatal visit, and after an initial negative urine culture, less than 1 percent of women develop urinary infection. Antepartum bacteriuria may persist after delivery and some women will demonstrate pyelo-graphic evidence of chronic infection, obstructive lesions, or congenital urinary abnormalities. Recurrent symptomatic infections are common. Most evidence indicates that it is unlikely that asymptomatic bacteriuria is a prominent factor in the genesis of low-birthweight or preterm infants. Controversy exists whether or not covert bacteriuria is associated with hypertension, preeclampsia, or maternal anemia. It is likely that asymptomatic bacteriuria has very little, if any, impact on pregnancy outcome except for serious urinary tract infections.


A clean-voided urine specimen containing more than 100,000 organisms per milliliter of a single uropathogen is diagnostic. It may be prudent to treat when lower concentrations are identified, because pyelonephritis develops in some women with colony counts of 20,000 to 50,000 organisms per milliliter.

The American Academy of Pediatrics and The American College of Obstetricians and Gynecologists (Guidelines for Perinatal Care, 6th ed AAP and ACOG 2007:100–1) recommend routine screening for bacteriuria at the first prenatal visit. Screening by urine culture may not be cost effective when the prevalence is low. For example, less expensive tests, such as the leukocyte esterasenitrite dipstick, have been shown to be cost effective with ASB prevalences of 2 percent or less. Susceptibility determination is not necessary because initial treatment is empirical.


Women with asymptomatic bacteriuria may be given treatment with any of the several antimicrobial regimens shown in Table 63-1. Selection of a particular antimicrobial can be on the basis of in vitro susceptibilities, but most often it is empirical. The recurrence rate for all of these regimens is about 30 percent. If asymptomatic bacteriuria goes untreated, approximately 25 percent of infected women will subsequently develop acute pyelonephritis. Eradication of bacteriuria with antimicrobial agents has been shown to prevent most of these clinically evident infections. For women with persistent or frequent bacteriuria recurrences, suppressive therapy for the remainder of pregnancy may be given with nitrofurantoin, 100 mg at bedtime.

TABLE 63-1. Antimicrobial Agents Used for Treatment of Pregnant Women with Asymptomatic Bacteriuria (ASB)



For further reading in Williams Obstetrics, 23rd ed.,

see Chapter 48, “Renal and Urinary Tract Disorders.”