What is the appropriate ampicillin dosing for Group B (beta‑hemolytic) Streptococcus infection in a newborn or young infant, in a pregnant woman requiring intrapartum prophylaxis, and in an adult patient?

Ampicillin Dosing for Group B Streptococcus Infection

For intrapartum prophylaxis in pregnant women, give ampicillin 2 g IV initially, then 1 g IV every 4 hours until delivery; for neonatal sepsis, use ampicillin as part of combination therapy with an aminoglycoside; for adult non-pregnant patients with GBS UTI, give ampicillin 2 g IV initially, then 1 g IV every 6 hours for 7-14 days. 1, 2, 3

Pregnant Women: Intrapartum Prophylaxis

The CDC recommends ampicillin 2 g IV as the initial dose, followed by 1 g IV every 4 hours until delivery for women requiring intrapartum GBS prophylaxis. 1, 2, 4

Key Clinical Context

• Ampicillin is an acceptable alternative to penicillin G (the preferred agent), though it has a broader spectrum of activity. 1, 2, 4
• Administer the first dose as soon as labor begins or membranes rupture—do not delay for screening results if risk factors are present. 1
• At least 4 hours of antibiotic exposure before delivery is required for maximum effectiveness, reducing early-onset neonatal GBS disease by 78%. 1, 4, 5
• When ampicillin is given less than 2 hours before delivery, vertical transmission rates remain high (29-46%), compared to only 1.2-2.9% when given more than 2 hours before delivery. 5

Indications for Intrapartum Prophylaxis

• Positive GBS vaginal-rectal culture at 36-37 weeks gestation. 1, 2
• GBS bacteriuria at any concentration during any trimester of the current pregnancy—this mandates both immediate UTI treatment and intrapartum prophylaxis during labor, regardless of prior treatment. 1, 2, 4
• Prior infant with invasive GBS disease. 1
• Unknown GBS status with labor before 37 weeks, membranes ruptured ≥18 hours, or intrapartum fever ≥38.0°C. 1

Special Pregnancy Scenarios

• Preterm labor (<37 weeks): Start ampicillin 2 g IV immediately at hospital admission; discontinue if not in true labor. 2
• Preterm premature rupture of membranes (PPROM): Give ampicillin 2 g IV once, then 1 g IV every 6 hours for at least 48 hours—this regimen provides both latency support and adequate GBS prophylaxis. 1, 2

Neonates with Suspected Sepsis

Any newborn with signs of sepsis should receive ampicillin IV as part of combination therapy with an aminoglycoside (typically gentamicin) to provide synergistic killing and coverage for both GBS and E. coli. 1, 4, 6

• The combination of ampicillin plus gentamicin demonstrates accelerated bacterial killing and improved survival compared to ampicillin alone in experimental models. 6
• Well-appearing newborns whose mothers had suspected chorioamnionitis should undergo limited evaluation and receive ampicillin therapy pending culture results. 1

Non-Pregnant Adults with GBS UTI

For symptomatic GBS urinary tract infections in non-pregnant adults, give ampicillin 2 g IV initially, then 1 g IV every 6 hours for 7-14 days depending on severity. 3

Critical Distinction

• Do NOT treat asymptomatic GBS bacteriuria in non-pregnant adults—this leads to unnecessary antibiotic exposure, resistance development, and adverse effects without clinical benefit. 2
• The only exception is patients scheduled for endoscopic urologic procedures involving mucosal trauma. 2

Penicillin-Allergic Patients

Low-Risk Allergy (no anaphylaxis, angioedema, respiratory distress, or urticaria)

• Cefazolin 2 g IV initially, then 1 g IV every 8 hours until delivery is the preferred alternative for intrapartum prophylaxis. 1, 2, 4
• Cross-reactivity between penicillins and cephalosporins occurs in approximately 10% of patients. 4

High-Risk Allergy

• Obtain clindamycin and erythromycin susceptibility testing immediately—clindamycin resistance ranges from 13-25% among GBS isolates. 2, 4
• If susceptible to both: Clindamycin 900 mg IV every 8 hours until delivery. 1, 2, 4
• If resistant or susceptibility unknown: Vancomycin 1 g IV every 12 hours until delivery. 1, 2, 4
• Perform D-zone testing on isolates that are erythromycin-resistant but clindamycin-susceptible to detect inducible clindamycin resistance. 1, 2

Common 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, 4, 3
• Do not assume that treating a GBS UTI during pregnancy eliminates the need for intrapartum prophylaxis—recolonization after oral antibiotics is typical, and intrapartum IV prophylaxis remains mandatory. 2
• Avoid underdosing or premature discontinuation—incomplete treatment leads to treatment failure or recurrence. 2, 3
• Do not delay medically necessary obstetric procedures solely to reach the 4-hour antibiotic exposure threshold—give prophylaxis as soon as labor begins. 4