What is the recommended intrapartum antibiotic regimen for a pregnant woman colonized with Group B Streptococcus (GBS)?

Intrapartum Antibiotic Prophylaxis for Group B Streptococcus in Pregnancy

For pregnant women colonized with Group B Streptococcus, intravenous penicillin G (5 million units initially, then 2.5-3.0 million units every 4 hours until delivery) is the recommended first-line antibiotic regimen administered during labor, not during pregnancy itself. 1

Critical Timing Principle: No Treatment Before Labor

• Oral or intravenous antibiotics given before labor are completely ineffective at eliminating GBS colonization and should never be used for asymptomatic vaginal colonization. 2 This is one of the most common and dangerous errors in GBS management.

• Treatment is administered only during active labor (intrapartum), not at any point during pregnancy when colonization is detected. 2, 3

• The goal is to achieve at least 4 hours of antibiotic administration before delivery for maximum effectiveness—this reduces early-onset neonatal GBS disease by 78-89%. 1, 3

Screening Protocol

• Universal vaginal-rectal screening should occur between 36 0/7 to 37 6/7 weeks' gestation using a single swab or two separate swabs. 2, 4

• The swab is collected without a speculum: first the lower vagina, then inserted through the anal sphincter into the rectum. 2

First-Line Intrapartum Regimen (No Penicillin Allergy)

• Penicillin G: 5 million units IV initially, then 2.5-3.0 million units IV every 4 hours until delivery 1, 2

  • This is preferred due to narrow spectrum, universal GBS susceptibility, and proven efficacy. 2, 5

• Ampicillin: 2 g IV initially, then 1 g IV every 4 hours until delivery 1, 2

  • Acceptable alternative but has broader spectrum activity. 2

Penicillin-Allergic Patients: Risk-Stratified Approach

Low-Risk Allergy (No History of Anaphylaxis, Angioedema, Respiratory Distress, or Urticaria)

• Cefazolin: 2 g IV initially, then 1 g IV every 8 hours until delivery 1, 2, 3
  • This is the preferred alternative for non-high-risk allergies. 2, 4
  • Note: Approximately 10% of penicillin-allergic patients have cross-reactivity to cephalosporins. 2

High-Risk Allergy (History of Anaphylaxis, Angioedema, Respiratory Distress, or Urticaria to Penicillin/Cephalosporin)

Critical step: Susceptibility testing for clindamycin and erythromycin must be performed on the GBS isolate. 1, 2, 4

• If susceptible to both clindamycin and erythromycin:

  • Clindamycin: 900 mg IV every 8 hours until delivery 1, 2

• If resistant to either agent or susceptibility unknown:

  • Vancomycin: 1 g IV every 12 hours until delivery 1, 2

• If susceptible to clindamycin but resistant to erythromycin:

  • Perform D-zone testing to detect inducible clindamycin resistance. 1, 2
  • Use clindamycin only if D-zone testing is negative. 1

Special Clinical Scenarios Requiring Intrapartum Prophylaxis

Automatic Indications (No Screening Needed)

• GBS bacteriuria at any concentration during any trimester of current pregnancy 2, 3, 4

  • These women require both immediate UTI treatment AND mandatory intrapartum prophylaxis during labor, regardless of whether the UTI was treated earlier. 2, 4
  • GBS bacteriuria indicates heavy genital tract colonization and significantly increases neonatal disease risk. 2

• Previous infant with invasive GBS disease 3, 4

Unknown GBS Status at Labor Onset

Administer intrapartum prophylaxis if any of the following:

• Labor begins before 37 weeks' gestation 2, 3, 4
• Membranes ruptured ≥18 hours 2, 3
• Intrapartum temperature ≥38.0°C (100.4°F) 2

Preterm Labor Management

• If preterm labor (<37 weeks) with unknown GBS status: obtain vaginal-rectal culture immediately and start GBS prophylaxis. 2, 4
• Discontinue prophylaxis if labor is successfully arrested and not in true labor. 3, 4

Preterm Premature Rupture of Membranes (PPROM)

• Ampicillin 2 g IV once, then 1 g IV every 6 hours for at least 48 hours provides both latency support and adequate GBS prophylaxis. 2

When Prophylaxis Is NOT Needed

• Planned cesarean delivery before labor onset with intact membranes, regardless of GBS colonization status. 3, 4
  • No routine GBS prophylaxis required in this scenario. 3

Key Resistance Patterns and Laboratory Considerations

• All GBS isolates remain universally susceptible to penicillin worldwide. 2, 5

• Clindamycin resistance ranges from 3-15% among GBS isolates. 2

• Erythromycin resistance is 7-21% among GBS isolates. 2

• Erythromycin is no longer an acceptable alternative for intrapartum GBS prophylaxis in penicillin-allergic women at high risk for anaphylaxis. 1

Critical Pitfalls to Avoid

• Never treat asymptomatic GBS vaginal colonization with oral or IV antibiotics before labor. This does not eliminate carriage, does not prevent neonatal disease, and promotes antibiotic resistance. 2, 6

• Do not assume that treating a GBS UTI during pregnancy eliminates the need for intrapartum prophylaxis. Recolonization after oral antibiotics is typical, making intrapartum IV prophylaxis mandatory. 2

• Do not delay medically necessary obstetric procedures to achieve 4 hours of prophylaxis. While 4 hours is optimal, procedures should not be delayed. 1

• Ensure laboratories are informed when urine specimens are from pregnant women so they report GBS at concentrations ≥10,000 CFU/mL. 2

• For high-risk penicillin allergy, clinicians must inform laboratories of the need for antimicrobial susceptibility testing. 1, 2 This must be requested within 3 days. 2

Effectiveness Data

• When administered ≥4 hours before delivery, intrapartum antibiotic prophylaxis is 78-89% effective in preventing early-onset neonatal GBS disease. 1, 2, 3

• Maternal GBS colonization increases neonatal disease risk 29-fold compared to non-colonized mothers. 2

• The risk of anaphylactic reaction to penicillin is approximately 5 per 10,000 treatments. 2, 6