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This is a clinician-written, evidence-based summary aligned to the USMLE Step 2 CK Content Outline. It is intended for medical students preparing for USMLE Step 2 CK. Management reflects current ACC/AHA, USPSTF, and APA guidelines. Always cross-reference with UpToDate, institutional protocols, and clinical judgment.
The Bottom Line
- Screen all pregnant patients at 36 0/7-37 6/7 weeks using rectovaginal culture
- Intrapartum antibiotic prophylaxis prevents early-onset neonatal GBS disease
- Penicillin G is first-line; ampicillin is an alternative
- Give prophylaxis regardless of culture if GBS bacteriuria in current pregnancy or prior infant with invasive GBS disease
- Unknown status in labor: prophylaxis for preterm labor, ROM >=18 hours, or intrapartum fever
Overview
GBS colonizes GI/genital tracts and can be transmitted during labor. Early-onset neonatal disease presents in first week with sepsis, pneumonia, or meningitis. Universal late-pregnancy screening plus intrapartum prophylaxis reduces early-onset disease.
Epidemiology
Maternal GBS colonization is common and usually asymptomatic. Neonatal risk is increased by heavy colonization, GBS bacteriuria, prior affected infant, preterm birth, prolonged ROM, intrapartum fever, and chorioamnionitis.
Clinical Features
Symptoms
Maternal colonization is asymptomatic
GBS UTI may cause dysuria or be asymptomatic
Intrapartum fever suggests intraamniotic infection
Neonatal disease: respiratory distress, temperature instability, poor feeding, lethargy, shock
Preterm labor or prolonged rupture increases risk
Signs
Positive rectovaginal culture at 36-37 weeks
GBS bacteriuria indicates heavy colonization
Maternal fever/uterine tenderness/fetal tachycardia suggests infection
Neonatal tachypnea, hypoxia, hypotension, apnea
Penicillin allergy history guides antibiotic choice
Investigations
First-line
Rectovaginal GBS cultureUniversal screening at 36 0/7-37 6/7 weeks
Urine cultureGBS bacteriuria requires intrapartum prophylaxis
Penicillin allergy assessmentDetermine low vs high anaphylaxis risk
Second-line
Clindamycin susceptibilityNeeded in high-risk allergy if clindamycin considered
Intrapartum NAATMay help if culture status unknown at term
Neonatal sepsis evaluationBlood culture and empiric antibiotics depending on symptoms/risk
Specialist
Infectious disease/neonatology consultationSevere neonatal disease or complex resistance/allergy
Allergy testingPenicillin allergy testing can clarify options
Management
ACOG Committee Opinion: Prevention of Group B Streptococcal Early-Onset Disease in Newborns1
Who receives prophylaxis
- Positive GBS culture in current pregnancy
- GBS bacteriuria during current pregnancy
- Prior infant with invasive GBS disease
- Unknown status with preterm labor, ROM >=18 hours, or intrapartum fever
- Positive intrapartum NAAT if used
2
Antibiotic choices
- Penicillin G IV first-line
- Ampicillin IV alternative
- Cefazolin for low-risk penicillin allergy
- Clindamycin only if high-risk allergy and isolate susceptible
- Vancomycin if high-risk allergy and clindamycin not usable
3
Special situations
- Planned cesarean before labor with intact membranes does not require GBS prophylaxis solely for colonization
- Do not delay necessary obstetric interventions solely to complete 4 hours of antibiotics
- Treat GBS bacteriuria during pregnancy if UTI/asymptomatic bacteriuria threshold met
Complications
- Early-onset neonatal sepsis: Often respiratory distress or shock
- Neonatal pneumonia: Common early manifestation
- Neonatal meningitis: Can occur, more typical late onset
- Maternal infection: UTI, chorioamnionitis, endometritis, wound infection
- Antibiotic allergy: Requires careful intrapartum selection
USMLE Step 2 CK Exam Tips
- 1GBS screening = 36 0/7-37 6/7 weeks rectovaginal culture
- 2First-line prophylaxis = IV penicillin G
- 3GBS bacteriuria = intrapartum prophylaxis regardless of later culture
- 4Prior infant with invasive GBS = prophylaxis regardless of culture
- 5Unknown GBS + ROM >=18 hours, fever, or preterm labor = prophylaxis
- 6Scheduled cesarean before labor with intact membranes does not need GBS prophylaxis solely for positive culture
- 7High-risk penicillin allergy with unknown/resistant clindamycin susceptibility = vancomycin
practicetest your knowledge on group b streptococcus in pregnancyApply what you've learnt with USMLE Step 2 CK-style questions from the iatroX Q-Bank — obstetrics & gynecology and beyond.
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