What oral ampicillin dose should be used to treat a Group B Streptococcus urinary‑tract infection in a pregnant woman in her second trimester?

Oral Ampicillin for GBS UTI in Second Trimester: Not Recommended

Oral ampicillin is not the appropriate treatment for Group B Streptococcus UTI in pregnancy; you should treat the acute UTI with standard pregnancy-safe oral antibiotics (such as nitrofurantoin or cephalexin), but the patient will still require intravenous intrapartum antibiotic prophylaxis during labor regardless of oral treatment now. 1

Critical Understanding: Two Separate Treatment Goals

The management of GBS bacteriuria in pregnancy involves two distinct phases that must both be addressed:

1. Immediate UTI Treatment (Now, at 2nd Trimester)

• Treat the acute UTI with standard oral antibiotics according to susceptibility testing, using pregnancy-safe agents such as nitrofurantoin, cephalexin, or amoxicillin based on local resistance patterns. 1, 2

• Oral ampicillin is not specifically recommended in current guidelines for outpatient UTI treatment in pregnancy, though it may be used if the organism is susceptible. 3, 4

• Nitrofurantoin is specifically recommended for GBS bacteriuria treatment in pregnancy by some experts due to excellent GBS susceptibility. 2

• Complete a standard 3-7 day course and obtain a test-of-cure urine culture 7 days after completing therapy. 4

2. Mandatory Intrapartum IV Prophylaxis (During Labor)

This is the critical point: Treating the UTI now does NOT eliminate the need for IV antibiotics during labor. 1

• All pregnant women with any concentration of GBS in urine at any point during pregnancy must receive IV intrapartum antibiotic prophylaxis during labor, regardless of whether the UTI was treated earlier. 1

• Oral antibiotics given before labor are completely ineffective at eliminating GBS colonization from the genital tract, and recolonization after oral treatment is typical. 1

• The intrapartum IV regimen is:

  • Penicillin G 5 million units IV initially, then 2.5-3.0 million units IV every 4 hours until delivery (preferred). 1
  • Ampicillin 2 g IV initially, then 1 g IV every 4 hours until delivery (acceptable alternative). 1

• Prophylaxis must be administered ≥4 hours before delivery for maximum effectiveness (78% reduction in early-onset neonatal GBS disease). 1

Why Oral Ampicillin Alone Is Insufficient

• GBS bacteriuria indicates heavy genital tract colonization and significantly increases the risk of early-onset neonatal GBS disease. 1

• Oral antibiotics do not achieve adequate concentrations in the genital tract to eliminate colonization, even if they cure the UTI. 1

• The CDC and ACOG explicitly state that antimicrobial agents should not be used before the intrapartum period to prevent GBS disease, as such treatment is ineffective and may promote resistance. 1

Special Pregnancy Scenarios

• If the patient develops preterm labor (<37 weeks): Initiate IV GBS prophylaxis immediately upon hospital admission. 1

• If preterm premature rupture of membranes (PPROM) occurs: Ampicillin 2 g IV once, followed by 1 g IV every 6 hours for at least 48 hours provides both latency prolongation and GBS prophylaxis. 1

Documentation Requirements

• Ensure that the finding of GBS bacteriuria is clearly documented in the prenatal record and communicated to the anticipated site of delivery. 1

• The patient does NOT need repeat GBS screening at 36-37 weeks, as she is presumed to be GBS-colonized and automatically qualifies for intrapartum prophylaxis. 1

Common Pitfalls to Avoid

• Do not assume that treating the UTI eliminates the need for intrapartum prophylaxis—this is a dangerous error that increases neonatal risk. 1

• Do not prescribe oral antibiotics with the intent of preventing neonatal GBS disease—only IV intrapartum prophylaxis is effective for this purpose. 1

• Do not use ampicillin empirically without susceptibility testing, as resistance rates to ampicillin among E. coli (the most common co-pathogen in pregnancy UTIs) are high. 3