Treatment of Asymptomatic Bacteriuria in Preterm Labor
Pregnant women with asymptomatic bacteriuria should be treated with 4-7 days of antimicrobial therapy, as this reduces the risk of pyelonephritis from 20-35% to 1-4% and decreases preterm delivery and low birth weight. 1
Screening and Diagnosis
- Screen all pregnant women with urine culture at the first prenatal visit to detect asymptomatic bacteriuria early in pregnancy 1
- Diagnosis requires ≥10^5 colony-forming units per mL of a single uropathogen on clean-catch midstream urine culture 2
- Standard urinalysis methods are insufficient; formal urine culture is mandatory 1
Evidence for Treatment
The recommendation to treat asymptomatic bacteriuria in pregnancy is based on robust, consistent evidence:
- Untreated asymptomatic bacteriuria increases pyelonephritis risk 20-30 fold compared to women without bacteriuria 1, 3
- Treatment reduces pyelonephritis rates from 20-35% down to 1-4% 1
- Antimicrobials reduce preterm birth risk from approximately 53 per 1000 to 14 per 1000 (risk difference -39 per 1000) 1
- Treatment reduces very low birth weight from 137 per 1000 to 88 per 1000 (risk difference -49 per 1000) 1
- Implementation of screening programs decreased pyelonephritis rates from 1.8-2.1% to 0.5-0.6% in pregnant populations 1, 3
Antibiotic Selection
First-line agents:
- Nitrofurantoin (safe and effective throughout pregnancy, except avoid at term due to hemolysis risk) 1, 3, 4
- Cephalexin or other first-generation cephalosporins (excellent safety profile, 500 mg four times daily) 3, 5
- Fosfomycin trometamol (single 3g dose or short course, safe and effective) 4, 6
Avoid:
- Ampicillin (high E. coli resistance rates) 5
- Trimethoprim/trimethoprim-sulfamethoxazole in first trimester (teratogenic potential) 3
- Fluoroquinolones throughout pregnancy 3
Treatment Duration
- Recommend 4-7 days of antimicrobial therapy rather than single-dose or shorter courses 1
- The optimal duration is antimicrobial-specific: nitrofurantoin and β-lactams require longer courses (4-7 days) as they are less effective with single-dose therapy 1
- Single-dose regimens show lower bacteriuria clearance rates and higher rates of low birth weight compared to 7-day courses 1
Follow-Up
- Obtain follow-up urine culture 1-2 weeks after completing treatment to confirm eradication 3
- Insufficient evidence exists to recommend routine repeat screening during pregnancy for women with initially negative cultures 1
- For recurrent infections, consider prophylactic antibiotics (cephalexin) for the remainder of pregnancy 3
Clinical Context
The strength of this recommendation (strong recommendation, moderate-quality evidence) is based on the consistency of benefit across multiple studies from the 1960s-1980s, despite their age 1. While a 2015 Dutch study suggested lower pyelonephritis rates (2.4%) in untreated low-risk women, this population was highly selected and enrolled women at low risk of complications 1. The guideline committee concluded that further evaluation in diverse populations is necessary before altering the universal screening and treatment recommendation 1.
The relationship between genital tract infection and preterm delivery is well-established through biochemical, microbiological, and clinical evidence 7. Treatment of asymptomatic bacteriuria is one of the few proven interventions to reduce infection-associated preterm birth 7, 8.