Asymptomatic bacteriuria complicates 2–10% of all pregnancies and is defined as bacterial colonization of the lower urinary tract without symptoms. The diagnosis has traditionally been established by two clean-catch urine specimens of > 100,000 colony-forming units (cfu)/mL but one midstream void of > 100,000 cfu/mL is now generally accepted as adequate for making the diagnosis. Many other screening tests for the detection of asymptomatic bacteriuria have been suggested, including urinalysis for the presence of nitrites and leukocyte esterases. This method was examined in a meta-analysis and found to have a high specificity (0.98) but a low sensitivity (0.46) in pregnant women for the detection of bacteriuria. Therefore, although the urine dipstick is effective at ruling out urinary infection, its usefulness at ruling in infection is limited and most guidelines instead recommend performing a single urine culture at the beginning of pregnancy. Some authorities recommend cultures in each trimester to improve detection rates.

As mentioned above, the incidence of asymptomatic bacteriuria is 2–10% but this varies with socio-economic status, parity, and sexual practice, as well as with certain medical conditions including sickle cell anemia and diabetes mellitus. The United States Preventative Services Task Force as well as several other international medical societies recommend screening for and treatment of asymptomatic bacteriuria. Screening and treatment of asymptomatic bacteriuria is cost-effective especially in populations where its incidence is greater than 2%. If untreated, up to 30% of cases will progress to pyelonephritis. Furthermore, asymptomatic bacteriuria has been associated with low birthweight and preterm birth. A review of randomized trials comparing antibiotic treatment versus no antibiotic treatment of asymptomatic bacteriuria revealed that the former management resulted in a greater clearance of bacteria as well as in a greater decrease in both pyelonephritis and low birthweight in babies. The rates of preterm delivery, however, were not affected by treatment. It should be noted that the overall quality of studies in this review was reported to be poor so caution in interpretation of these studies must be maintained. With proper treatment of asymptomatic bacteriuria, the number needed to treat to prevent one episode of pyelonephritis is only 7, and the rate of hospitalization for pyelonephritis is reduced to 1.4%.

As in the nonpregnant state, E. coli is the most common uropathogen found in asymptomatic bacteriuria, accounting for about 80% of isolates. Other pathogens include K. pneumoniae, Enterococcus species, S. saprophyticus, and P. mirabilis. Finally, group B hemolytic streptococcus (GBS) in the urine has been associated with preterm rupture of membranes and preterm delivery, as well as with early-onset neonatal sepsis and postpartum chorio-amnionitis. Antepartum treatment of GBS significantly decreases the above-mentioned complications. Further intrapartum prophylaxis for antepartum GBS bacteriuria is also recommended to reduce the postpartum and neonatal complications mentioned above.

A meta-analysis of 10 randomized controlled trials comparing single dose versus 4–7-day treatments of asymptomatic bacteriuria showed a nonstatistically significant higher “no cure rate” for those in the single-dose group. There were no significant differences in the rates of recurrence, preterm delivery or pyelonephritis but the studies were not sufficiently powered to detect differences in the latter two outcomes. Finally, single-dose treatments were associated with fewer adverse side effects of any kind. The study concluded that there was not enough evidence to support the use of one regimen over the other but single-dose treatment was associated with lower costs and side effects. Therefore, the recommendations for treatment of asymptomatic bacteriuria at present remain standard cystitis treatment regimens.