Treatment of Asymptomatic Bacteriuria in Pregnancy
Asymptomatic bacteriuria in pregnancy must always be treated with antibiotics—this is the one clinical scenario where asymptomatic bacteriuria requires treatment. 1, 2
Why Treatment is Mandatory
The evidence for treating asymptomatic bacteriuria in pregnancy is compelling and based on clear morbidity outcomes:
Untreated asymptomatic bacteriuria carries a 20-35% risk of progression to pyelonephritis, which drops to only 1-4% with antibiotic treatment—representing a 20-30 fold risk reduction. 2, 3
Treatment reduces preterm birth from approximately 53 per 1000 to 14 per 1000 pregnancies. 2
Antimicrobial therapy reduces very low birth weight from approximately 137 per 1000 to 88 per 1000 infants. 2
Implementation of screening programs decreased pyelonephritis rates from 1.8-2.1% to 0.5-0.6%. 1
These are not theoretical risks—these are concrete improvements in maternal and neonatal morbidity that justify universal screening and treatment. 4
Screening Approach
Obtain a urine culture at 12-16 weeks gestation or at the first prenatal visit if later. 1, 3
Do not rely on urinalysis or pyuria alone—screening for pyuria has only 50% sensitivity for identifying bacteriuria. 1, 3
Asymptomatic bacteriuria is defined as ≥10^5 CFU/mL on urine culture without symptoms. 3
Asymptomatic bacteriuria occurs in 2-7% of pregnant women, making screening cost-effective. 2, 5
First-Line Antibiotic Options
For first trimester:
- Nitrofurantoin 50-100 mg four times daily is the preferred first-line agent. 1
- Fosfomycin 3g single dose is an acceptable alternative. 1
- Cephalexin 500 mg four times daily is appropriate if nitrofurantoin is contraindicated. 1
For third trimester:
- Cephalexin 500 mg four times daily is preferred (avoid nitrofurantoin near term due to theoretical hemolysis risk). 1
- Amoxicillin-clavulanate 20-40 mg/kg per day in 3 divided doses if the pathogen is susceptible. 1
Antibiotics to avoid:
- Trimethoprim-sulfamethoxazole in the first trimester (teratogenic) and contraindicated in the third trimester. 1
- Fluoroquinolones throughout pregnancy (fetal cartilage concerns). 1
Treatment Duration
The recommended treatment duration is 4-7 days for asymptomatic bacteriuria. 2, 3 While some sources mention 7-14 days for symptomatic UTIs, the American College of Obstetricians and Gynecologists specifically recommends 4-7 days for asymptomatic bacteriuria. 2
- Single-dose regimens show lower clearance rates and are not recommended. 2
Critical Follow-Up
Obtain a follow-up urine culture 1-2 weeks after completing treatment to confirm clearance. 1
Continue periodic screening with urine cultures throughout the remainder of pregnancy after any treated episode. 2
Recurrence is common and each recurrence requires re-treatment with another 4-7 day course. 2
Special Consideration: Group B Streptococcus
- If GBS bacteriuria is detected at any concentration during pregnancy, treat at the time of diagnosis AND provide intrapartum GBS prophylaxis during labor—GBS bacteriuria indicates heavy genital tract colonization. 1
Common Pitfalls to Avoid
Do not ignore asymptomatic bacteriuria in pregnancy thinking it's benign—pregnancy is the exception to the general rule of not treating asymptomatic bacteriuria. 2, 6
Do not use agents like nitrofurantoin for suspected pyelonephritis—they don't achieve adequate blood concentrations. 1
Do not treat based on pyuria alone without a positive culture. 3
Do not fail to obtain follow-up cultures—recurrence rates are significant and require detection and re-treatment. 2