Why is penicillin administered to pregnant women with Group B streptococcal (GBS) colonization during labor?

Why Penicillin is Administered to GBS-Positive Mothers During Labor

Intrapartum penicillin prophylaxis prevents life-threatening early-onset neonatal Group B streptococcal infections by reducing vertical transmission from colonized mothers to their newborns during delivery, achieving an 86-89% reduction in early-onset GBS disease when administered at least 4 hours before birth. 1

The Core Problem: Vertical Transmission Risk

• Approximately 25% of pregnant women are colonized with GBS in their genital or rectal areas, creating a reservoir for potential neonatal infection during passage through the birth canal 2
• Maternal GBS colonization increases the risk of neonatal disease 29-fold compared to non-colonized mothers 3
• Without prophylaxis, vertical transmission during labor and delivery can lead to devastating neonatal outcomes including meningitis, sepsis, and death 2

Mechanism of Prevention

• Intravenous antibiotics during labor reduce maternal GBS bacterial load at the time of delivery, preventing transmission to the newborn 4
• The antibiotics must be administered at least 4 hours before delivery to achieve adequate drug levels in fetal circulation and amniotic fluid 4
• When given ≥4 hours before delivery, prophylaxis is 78% effective in preventing early-onset neonatal GBS disease 3
• Shorter durations of ≥2 hours confer some protection but with reduced efficacy 2

Why Penicillin Specifically

• Penicillin G is the first-line agent due to its narrow antimicrobial spectrum, which minimizes disruption of normal flora and reduces selection pressure for antibiotic-resistant organisms 1, 2
• The recommended regimen is penicillin G 5 million units IV initially, then 2.5-3.0 million units IV every 4 hours until delivery 4, 1
• All GBS isolates remain universally susceptible to penicillin, ensuring reliable efficacy 5
• Ampicillin (2 g IV initially, then 1 g IV every 4 hours) is an acceptable alternative but has broader spectrum activity 1

Evidence of Effectiveness

• The incidence of invasive early-onset GBS disease decreased by more than 80% from 1.8 cases/1000 live births in the early 1990s to 0.26 cases/1000 live births in 2010 6
• From 1994 to 2010, over 70,000 cases of early-onset GBS invasive disease were prevented in the United States through implementation of screening and prophylaxis strategies 6
• IAP effectiveness is similarly high among term (91%) and preterm (86%) infants when first-line therapy is received for at least 4 hours 6

Who Receives Prophylaxis

All pregnant women with positive GBS vaginal-rectal screening culture at 35-37 weeks' gestation must receive intrapartum antibiotic prophylaxis during labor 4

Additional indications include:

• GBS bacteriuria at any concentration during the current pregnancy 4
• Previous infant with invasive GBS disease 4
• Unknown GBS status at labor onset with delivery <37 weeks' gestation, membrane rupture ≥18 hours, or intrapartum temperature ≥100.4°F (≥38.0°C) 4

Critical Timing Considerations

• Antibiotics must be initiated as soon as labor begins or membranes rupture to maximize the likelihood of achieving ≥4 hours of prophylaxis before delivery 1, 2
• The most frequent reason for inadequate prophylaxis is less than 4 hours from antibiotic administration to birth, occurring in approximately 30% of cases despite adequate time in the hospital 7
• Medically necessary obstetric procedures should not be delayed to achieve 4 hours of prophylaxis, but timing should be optimized when possible 4

Why NOT Oral Antibiotics Before Labor

• Oral or IV antibiotics given before the intrapartum period are completely ineffective at eliminating GBS colonization and should never be used for asymptomatic vaginal colonization 3, 2
• Prenatal treatment does not prevent neonatal disease because recolonization after antibiotics is typical 3
• Such treatment may cause adverse consequences including antibiotic resistance without providing benefit 4

Penicillin Allergy Alternatives

For women with penicillin allergy not at high risk for anaphylaxis:

• Cefazolin 2 g IV initially, then 1 g IV every 8 hours until delivery 1, 2

For women at high risk for anaphylaxis (history of immediate hypersensitivity reactions):

• Clindamycin 900 mg IV every 8 hours if the isolate is susceptible 1, 2
• Vancomycin 1 g IV every 12 hours if clindamycin resistance or susceptibility unknown 1, 2
• Susceptibility testing must be performed on GBS isolates from penicillin-allergic women at high risk for anaphylaxis 3

Common Pitfalls to Avoid

• Never assume that treating a GBS UTI earlier in pregnancy eliminates the need for intrapartum prophylaxis—women with GBS bacteriuria at any point during pregnancy automatically qualify for IAP during labor 3
• Do not administer prophylaxis to women undergoing planned cesarean delivery before labor onset with intact membranes, regardless of GBS colonization status 4
• Erythromycin is no longer an acceptable alternative for intrapartum GBS prophylaxis 4
• The screening culture remains valid for only 5 weeks, so women who deliver >5 weeks after screening may need risk-based management 2