What is the recommended treatment for an asymptomatic urinary tract infection (UTI) in a pregnant woman?

Treatment of Asymptomatic Bacteriuria in Pregnancy

All pregnant women should be screened for asymptomatic bacteriuria (ASB) with a urine culture early in pregnancy, and if detected, it must be treated with 4-7 days of antibiotics to prevent pyelonephritis and adverse pregnancy outcomes. 1

Why Treatment is Essential

Pregnancy is the only clinical scenario where asymptomatic bacteriuria requires mandatory treatment, unlike in non-pregnant populations where ASB is typically left untreated. 1, 2 The evidence supporting this approach is compelling:

• Untreated ASB increases pyelonephritis risk 20-30 fold, from baseline rates of 20-35% down to 1-4% with treatment 1, 2
• Treatment reduces preterm birth from approximately 53 per 1000 to 14 per 1000 pregnancies (moderate-quality evidence) 1
• Treatment reduces very low birth weight from approximately 137 per 1000 to 88 per 1000 infants (moderate-quality evidence) 1
• Implementation of screening programs decreased pyelonephritis rates from 1.8-2.1% to 0.5-0.6% 2

Screening Approach

• Obtain a single urine culture at 12-16 weeks gestation or at the first prenatal visit if presenting later 2, 3
• Do not rely on urinalysis or dipstick alone - pyuria screening has only 50% sensitivity for detecting bacteriuria in pregnancy 2, 3
• Urine culture is the gold standard and mandatory for diagnosis 2, 3

First-Line Antibiotic Options

Preferred Agents:

Nitrofurantoin is the first-line choice across all trimesters (except avoid near term):

• Dose: 100 mg orally twice daily for 4-7 days 2, 3
• Excellent safety profile with minimal teratogenic risk 3
• Achieves adequate urinary concentrations 3

Fosfomycin trometamol is an excellent alternative:

• Single 3-gram dose 2, 4
• Particularly convenient for compliance 2
• Acceptable throughout pregnancy 2

Cephalexin as a safe alternative:

• 500 mg four times daily for 4-7 days 2, 3
• Excellent safety profile in pregnancy 2
• Achieves adequate blood and urinary concentrations 2

Other Acceptable Cephalosporins:

• Cefpodoxime, cefuroxime, or cefixime may be used 2, 5

Treatment Duration

The recommended duration is 4-7 days rather than shorter courses, though the optimal duration remains somewhat uncertain. 1, 2 While Cochrane reviews found insufficient evidence comparing single-dose, 3-day, or 4-day regimens to 7-day courses, the consistency of benefit with slightly longer courses supports this recommendation. 1

Critical Antibiotics to AVOID

• Trimethoprim and trimethoprim-sulfamethoxazole: Contraindicated in first trimester due to neural tube defect risk from folic acid interference; also contraindicated in third trimester 2, 3
• Fluoroquinolones (ciprofloxacin, levofloxacin): Avoid throughout entire pregnancy due to potential fetal cartilage damage 2, 3
• Nitrofurantoin near term: Should not be used close to delivery due to theoretical risk of hemolytic anemia in the newborn 2

Essential Follow-Up

• Obtain follow-up urine culture 1-2 weeks after completing treatment to confirm bacteriologic cure 2, 3
• Do not perform repeated surveillance cultures or treat ASB repeatedly after the initial screen-and-treat approach, as this fosters antimicrobial resistance 2
• There is insufficient evidence to recommend routine repeat screening during pregnancy for women with an initial negative culture 1

Special Consideration: Group B Streptococcus

If GBS is detected in urine at any concentration during pregnancy:

• Treat the bacteriuria immediately with appropriate antibiotics 2
• The woman automatically qualifies for intrapartum antibiotic prophylaxis during labor - no need for vaginal-rectal screening at 35-37 weeks 2, 6
• GBS bacteriuria indicates heavy genital tract colonization 2

Important Caveat: Lactobacillus

If urine culture grows Lactobacillus species, do not treat. 6 Lactobacillus is normal vaginal/periurethral flora and represents contamination rather than true bacteriuria. 6 Only treat true uropathogens (E. coli, Klebsiella, Proteus, Enterococcus, GBS). 6

Common Pitfalls to Avoid

• Don't skip the urine culture - empiric treatment without culture confirmation and susceptibility testing is inadequate 2, 3
• Don't use urinalysis alone to rule out bacteriuria - it misses 50% of cases 2, 3
• Don't classify pregnant women with UTI as "complicated" unless they have structural/functional urinary tract abnormalities or immunosuppression, as this leads to unnecessary broad-spectrum antibiotic overuse 2
• Don't ignore the evidence - while a 2015 Dutch study suggested non-treatment might be acceptable in selected low-risk women, the IDSA guideline committee felt further evaluation in other populations was necessary before changing the strong recommendation to screen and treat 1