Pathogenesis

Pregnant women are at increased risk for UTIs. Beginning in week 6 and peaking during weeks 22 to 24, approximately 90 percent of pregnant women develop ureteral dilatation, which will remain until delivery (hydronephrosis of pregnancy). Increased bladder volume and decreased bladder tone, along with decreased ureteral tone, contribute to increased urinary stasis and ureterovesical reflux.1 Additionally, the physiologic increase in plasma volume during pregnancy decreases urine concentration. Up to 70 percent of pregnant women develop glycosuria, which encourages bacterial growth in the urine. Increases in urinary progestins and estrogens may lead to a decreased ability of the lower urinary tract to resist invading bacteria. This decreased ability may be caused by decreased ureteral tone or possibly by allowing some strains of bacteria to selectively grow.1,3These factors may all contribute to the development of UTIs during pregnancy.

Bacteriology

The organisms that cause UTIs during pregnancy are the same as those found in nonpregnant patients. Escherichia coli accounts for 80 to 90 percent of infections. Other gram-negative rods such as Proteus mirabilis and Klebsiella pneumoniae are also common. Gram-positive organisms such as group B streptococcus and Staphylococcus saprophyticus are less common causes of UTI. Group B streptococcus has important implications in the management of pregnancy and will be discussed further. Less common organisms that may cause UTI include enterococci, Gardnerella vaginalis and Ureaplasma ureolyticum.1,4,5

Diagnosis and Treatment of UTIs

UTIs have three principle presentations: asymptomatic bacteriuria, acute cystitis and pyelonephritis. The diagnosis and treatment of UTI depends on the presentation.

ASYMPTOMATIC BACTERIURIA

[corrected] Significant bacteriuria may exist in asymptomatic patients. In the 1960s, Kass6 noted the subsequent increased risk of developing pyelonephritis in patients with asymptomatic bacteriuria. Significant bacteriuria has been historically defined as finding more than 10⁵ colony-forming units per mL of urine.7 Recent studies of women with acute dysuria have shown the presence of significant bacteriuria with lower colony counts. This has not been studied in pregnant women, and finding more than 10⁵ colony-forming units per mL of urine remains the commonly accepted standard. Asymptomatic bacteriuria is common, with a prevalence of 10 percent during pregnancy.6,8 Thus, routine screening for bacteriuria is advocated.

Untreated asymptomatic bacteriuria leads to the development of symptomatic cystitis in approximately 30 percent of patients and can lead to the development of pyelonephritis in up to 50 percent.6 Asymptomatic bacteriuria is associated with an increased risk of intra-uterine growth retardation and low-birth-weight infants.9 The relatively high prevalence of asymptomatic bacteriuria during pregnancy, the significant consequences for women and for the pregnancy, plus the ability to avoid sequelae with treatment, justify screening pregnant women for bacteriuria.

SCREENING

The American College of Obstetrics and Gynecology recommends that a urine culture be obtained at the first prenatal visit.10 A repeat urine culture should be obtained during the third trimester, because the urine of treated patients may not remain sterile for the entire pregnancy.10 The recommendation of the U.S. Preventative Services Task Force is to obtain a urine culture between 12 and 16 weeks of gestation (an “A” recommendation).11

By screening for and aggressively treating pregnant women with asymptomatic bacteriuria, it is possible to significantly decrease the annual incidence of pyelonephritis during pregnancy.8,12 In randomized controlled trials, treatment of pregnant women with asymptomatic bacteriuria has been shown to decrease the incidence of preterm birth and low-birth-weight infants.13

Rouse and colleagues14 performed a cost-benefit analysis of screening for bacteriuria in pregnant women versus inpatient treatment of pyelonephritis and found a substantial decrease in overall cost with screening. The cost of screening for bacteriuria to prevent the development of pyelonephritis in one patient was $1,605, while the cost of treating one patient with pyelonephritis was $2,485. Wadland and Plante15 performed a similar analysis in a family practice obstetric population and found screening for asymptomatic bacteriuria to be cost-effective.

The decision about how to screen asymptomatic women for bacteriuria is a balance between the cost of screening versus the sensitivity and specificity of each test. The gold standard for detection of bacteriuria is urine culture, but this test is costly and takes 24 to 48 hours to obtain results. The accuracy of faster screening methods (e.g., leukocyte esterase dipstick, nitrite dipstick, urinalysis and urine Gram staining) has been evaluated (Table 116). Bachman and associates16 compared these screening methods with urine culture and found that while it was more cost effective to screen for bacteriuria with the esterase dipstick for leukocytes, only one half of the patients with bacteriuria were identified compared with screening by urine culture. The increased number of false negatives and the relatively poor predictive value of a positive test make the faster methods less useful; therefore, a urine culture should be routinely obtained in pregnant women to screen for bacteriuria at the first prenatal visit and during the third trimester.10,11