What is the recommended approach for Group B (Streptococcus) prophylaxis in the third trimester?

Group B Streptococcus Prophylaxis in the Third Trimester

The recommended approach for Group B Streptococcus (GBS) prophylaxis in the third trimester is universal screening at 35-37 weeks gestation with vaginal-rectal cultures, followed by intrapartum antibiotic prophylaxis for all GBS-positive women during labor. 1, 2

Screening Protocol

• Timing: Screen all pregnant women at 35-37 weeks gestation (optimal window is 36 0/7 to 37 6/7 weeks) 3
• Method: Single swab collection from lower vagina followed by rectum (through anal sphincter) 1
• Special populations requiring screening:
  • Women with planned cesarean delivery (due to risk of labor or membrane rupture before scheduled surgery) 4
  • Women with previous GBS treatment during the current pregnancy 2

Indications for Intrapartum Antibiotic Prophylaxis

Intrapartum antibiotic prophylaxis is indicated for:

1. Positive GBS vaginal/rectal culture at 35-37 weeks gestation
2. GBS bacteriuria during current pregnancy (regardless of colony count)
3. Previous infant with invasive GBS disease
4. Unknown GBS status with:
   • Delivery at <37 weeks gestation
   • Rupture of membranes ≥18 hours
   • Intrapartum fever ≥38°C (100.4°F)

Recommended Antibiotic Regimens

First-line therapy 1, 2:

• Penicillin G: 5 million units IV initial dose, then 2.5-3.0 million units IV every 4 hours until delivery
• OR Ampicillin: 2 g IV initial dose, then 1 g IV every 4 hours until delivery

For penicillin-allergic patients 1:

• Without history of anaphylaxis/angioedema/respiratory distress:
  • Cefazolin: 2 g IV initial dose, then 1 g IV every 8 hours until delivery
• With history of anaphylaxis/angioedema/respiratory distress:
  • If GBS isolate is susceptible: Clindamycin 900 mg IV every 8 hours until delivery
  • If GBS isolate is resistant or susceptibility unknown: Vancomycin 1 g IV every 12 hours until delivery

Management of Special Situations

Preterm Labor (<37 weeks)

For women presenting with signs/symptoms of preterm labor 1:

1. Obtain vaginal-rectal GBS culture
2. Start GBS prophylaxis
3. If determined not to be in true labor, discontinue prophylaxis
4. If GBS culture results become available and are negative, discontinue prophylaxis
5. If patient remains pregnant and reaches 35-37 weeks, repeat screening

Preterm Premature Rupture of Membranes (PPROM)

For women with PPROM 1:

1. Obtain vaginal-rectal GBS culture
2. Start antibiotics for latency or GBS prophylaxis
3. If entering labor, continue antibiotics until delivery
4. If not in labor, continue antibiotics per standard of care for latency or for 48 hours if receiving for GBS prophylaxis

Important Clinical Considerations

• Duration of prophylaxis: While 4 hours of antibiotic administration before delivery is optimal, even shorter durations (≥2 hours) have been shown to reduce GBS vaginal colony counts and decrease neonatal sepsis 3
• Pharmacokinetics: Studies show that even with <4 hours of prophylaxis, fetal serum penicillin G levels significantly exceed the minimal inhibitory concentration for GBS, suggesting benefit even in precipitous labors 5
• Oral antibiotics ineffective: Oral antimicrobial agents should not be used to treat GBS colonization during pregnancy as they are ineffective in eliminating carriage or preventing neonatal disease 2
• Previous treatment not sufficient: Women who received treatment for GBS UTI during pregnancy still require intrapartum prophylaxis, as prior treatment does not eliminate the risk of colonization at delivery 2

Common Pitfalls to Avoid

1. Treating GBS colonization with oral antibiotics in the third trimester: This approach is ineffective as studies show that 30-70% of treated women remain colonized at delivery 1
2. Skipping intrapartum prophylaxis for women previously treated for GBS UTI: These women still require intrapartum prophylaxis as recolonization is common 2
3. Delaying necessary obstetric interventions to achieve 4 hours of prophylaxis: Medically necessary interventions should not be delayed solely to provide 4 hours of antibiotics 3
4. Failing to perform susceptibility testing for penicillin-allergic patients: This is essential due to increasing resistance to alternative antibiotics 2

By following these evidence-based guidelines, the risk of early-onset GBS disease can be significantly reduced, improving neonatal outcomes and reducing morbidity and mortality.