Macrobid (Nitrofurantoin) for Group B Streptococcus in Pregnancy
Macrobid (nitrofurantoin) is NOT recommended for Group B Streptococcus treatment or prophylaxis in pregnant women. The CDC explicitly states that intravenous penicillin G or ampicillin are the only proven effective agents for preventing early-onset neonatal GBS disease, and oral antibiotics including nitrofurantoin have been shown ineffective at eliminating GBS colonization or preventing neonatal disease 1.
Why Nitrofurantoin Fails for GBS
The fundamental problem is that oral antibiotics cannot eradicate GBS colonization during pregnancy. The CDC guidelines explicitly state that "oral antimicrobial agents should not be used to treat women who are found to be colonized with GBS during prenatal screening. Such treatment is not effective in eliminating carriage or preventing neonatal disease" 1. This applies to all oral antibiotics, including nitrofurantoin 2.
- Even if GBS is treated with oral antibiotics earlier in pregnancy, recolonization occurs rapidly, making prenatal treatment futile 1
- The only effective prevention strategy is intravenous antibiotics given during active labor (intrapartum prophylaxis), which achieves adequate levels in fetal circulation and amniotic fluid 1
The Exception: GBS Urinary Tract Infection
If a pregnant woman has GBS bacteriuria (GBS in urine), this represents an actual urinary tract infection that requires treatment according to standard UTI protocols 1, 2. However, even in this scenario:
- The UTI should be treated with appropriate antibiotics per standard UTI guidelines (which may include nitrofurantoin if appropriate for the specific clinical situation) 2
- Critically, treating the GBS UTI does NOT eliminate the need for intrapartum prophylaxis during labor 2, 3
- Women with GBS bacteriuria at any concentration during pregnancy must still receive IV penicillin or ampicillin during labor regardless of prior treatment 2, 4
Correct Management of GBS in Pregnancy
For GBS Colonization (Positive Vaginal-Rectal Culture)
Do NOT treat with any antibiotics before labor begins 1, 2. Instead:
- Screen all pregnant women at 35-37 weeks gestation with vaginal-rectal culture 1, 4
- If positive, administer intravenous penicillin G (5 million units initially, then 2.5 million units every 4 hours) during active labor until delivery 1, 2
- Alternative: IV ampicillin (2 g initially, then 1 g every 4 hours) 1, 2
- Treatment must be given for at least 4 hours before delivery for maximum effectiveness (86-89% reduction in early-onset disease) 1, 5
For Penicillin-Allergic Patients
Risk stratification is essential 2:
- Low-risk allergy (no anaphylaxis history): Use cefazolin 2 g IV initially, then 1 g IV every 8 hours 2
- High-risk allergy (history of anaphylaxis, angioedema, urticaria, or asthma): Use clindamycin 900 mg IV every 8 hours IF susceptibility testing confirms sensitivity, otherwise vancomycin 1 g IV every 12 hours 2
Critical Pitfalls to Avoid
- Never prescribe oral antibiotics (including Macrobid) to "treat" GBS colonization found on prenatal screening - this is ineffective and may cause unnecessary antibiotic resistance and adverse effects 1, 2
- Do not assume that treating GBS bacteriuria eliminates the need for intrapartum prophylaxis - both are required 2, 3
- Ensure IV antibiotics are started early enough in labor - ideally at least 4 hours before delivery for optimal protection 1, 5
- Remember that approximately 10% of penicillin-allergic patients also react to cephalosporins - careful allergy history is essential 1, 2
The Evidence Base
The recommendation against oral antibiotics is based on strong evidence showing that intravenous intrapartum prophylaxis reduces early-onset GBS disease by 80-95%, while oral antenatal antibiotics have no proven efficacy 1, 6, 5. Clinical trials in the 1980s demonstrated 100% efficacy for IV penicillin or ampicillin given during labor, whereas other strategies including oral antibiotics failed to prevent neonatal disease 1.