What is the recommended protocol for Group B Streptococcus screening at 35–37 weeks gestation and intrapartum antibiotic prophylaxis, including regimens for penicillin‑allergic patients?

Group B Streptococcus Screening and Intrapartum Antibiotic Prophylaxis

Universal Screening Protocol

All pregnant women should undergo vaginal-rectal GBS screening between 36 0/7 and 37 6/7 weeks of gestation using a single swab collected first from the lower vaginal introitus, then inserted through the anal sphincter into the rectum. 1

• The CDC previously recommended screening at 35-37 weeks, but ACOG now specifies 36 0/7-37 6/7 weeks as the optimal window 2, 1
• Use a single swab (or two separate swabs) without a speculum—cervical, perianal, perirectal, or perineal specimens are unacceptable 2, 3
• Place swabs in non-nutritive transport medium (Stuart's or Amies with or without charcoal) and refrigerate if processing is delayed 2, 3
• Laboratories should incubate specimens 1-4 hours at 35-37°C in enrichment broth, then subculture onto blood agar to maximize GBS recovery 2, 3
• A negative screen remains valid for only 5 weeks; beyond this, rescreening is required 3, 4

Indications for Intrapartum Antibiotic Prophylaxis

Administer IV antibiotics during labor to women with:

• Positive GBS vaginal-rectal culture at 36 0/7-37 6/7 weeks 1, 4
• GBS bacteriuria at any concentration during any trimester of current pregnancy (regardless of whether the UTI was treated) 2, 5, 6
• Previous infant with invasive GBS disease 2, 3, 6
• Unknown GBS status at labor onset with delivery <37 weeks' gestation 2, 3
• Unknown GBS status with membrane rupture ≥18 hours 2, 3
• Unknown GBS status with intrapartum temperature ≥38.0°C (100.4°F) 2, 3

Do not routinely give prophylaxis for planned cesarean delivery before labor onset with intact membranes, even if GBS-positive. 4, 1

Antibiotic Regimens

For Women Without Penicillin Allergy

Penicillin G remains the agent of choice: 5 million units IV initially, then 2.5-3.0 million units IV every 4 hours until delivery 2, 5, 4

• Ampicillin is an acceptable alternative: 2 g IV initially, then 1 g IV every 4 hours until delivery 2, 5, 4
• Penicillin G is preferred due to its narrow spectrum and universal GBS susceptibility 2, 5
• Prophylaxis should be administered ≥4 hours before delivery for maximum effectiveness (78% reduction in early-onset disease), though even 2 hours provides benefit 5, 1

For Penicillin-Allergic Women

Risk stratification is essential: Women with history of anaphylaxis, angioedema, respiratory distress, or urticaria after penicillin or cephalosporin are considered high-risk for anaphylaxis 2

For 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 2, 5, 4

For high-risk allergy:

• Order clindamycin and erythromycin susceptibility testing on the antenatal GBS isolate 2, 5, 6
• If susceptible to both clindamycin and erythromycin: Clindamycin 900 mg IV every 8 hours until delivery 2, 5, 4
• If resistant to either antibiotic or susceptibility unknown: Vancomycin 1 g IV every 12 hours until delivery 2, 5, 4
• Perform D-zone testing on isolates that are erythromycin-resistant but clindamycin-susceptible to detect inducible clindamycin resistance 2, 5

Special Clinical Scenarios

Preterm Labor or PPROM (<37 weeks)

• Screen immediately for GBS unless a negative culture was obtained within the prior 5 weeks 3, 6
• Initiate IV GBS prophylaxis at hospital admission while awaiting culture results 5, 3
• Continue prophylaxis for minimum 48 hours or discontinue if not in true labor and culture returns negative 5, 6
• For PPROM ≥24 weeks: Ampicillin 2 g IV once, then 1 g IV every 6 hours for at least 48 hours provides both latency prolongation and GBS prophylaxis 5

GBS Bacteriuria in Pregnancy

• Treat the acute UTI immediately according to standard pregnancy protocols 5, 6
• Critical: Treating the UTI does NOT eliminate GBS colonization—intrapartum IV prophylaxis remains mandatory regardless of when the UTI was treated 5
• Do not rescreen these women at 36-37 weeks; they automatically qualify for intrapartum prophylaxis 5
• Laboratories should report GBS in urine at concentrations ≥10,000 CFU/mL 2, 5

Critical Pitfalls to Avoid

Never treat asymptomatic GBS vaginal colonization with oral or IV antibiotics before labor—this is completely ineffective at eliminating carriage, does not prevent neonatal disease, and promotes antibiotic resistance. 2, 5, 4

• Approximately 10% of penicillin-allergic patients also react to cephalosporins, making allergy history assessment crucial 5
• Clindamycin resistance ranges 3-15% and erythromycin resistance 7-21% among GBS isolates, necessitating susceptibility testing for high-risk allergic patients 5
• Obstetric interventions should not be delayed solely to achieve 4 hours of antibiotic administration, as even 2 hours provides benefit 1
• Studies show inadequate screening (51.5% in one cohort) and incomplete IAP administration (50% in another) contribute to preventable neonatal infections 7, 8