Obstetric · Infection
GBS Intrapartum Prophylaxis
Group B Strep (GBS) intrapartum antibiotic prophylaxis dosing — first-line penicillin G plus allergy alternatives. Per ACOG CO 797 (2024) and CDC GBS 2024.
Last reviewed 26 May 2026
GBS Intrapartum Prophylaxis
Which antibiotic and what dose?
Penicillin allergy
Recommended regimen
Penicillin G (first line)
Loading dose
5 million units IV
Maintenance
2.5–3 million units IV every 4 hours until delivery
Alternative: Ampicillin 2 g IV loading, then 1 g IV every 4 h.
Adequate prophylaxis = first dose of penicillin, ampicillin, or cefazolin ≥ 4 hours before delivery. Clindamycin and vancomycin are NOT counted as adequate by Kaiser EOS / AAP for newborn-side decisions (insufficient placental transfer data).
Educational tool only — not medical advice. ACOG CO 797 (2020 reaffirmed 2024) + CDC GBS 2024 + RCOG GTG 36. Indications for IAP: positive 36–37+6 wk vaginal/rectal screen, GBS bacteriuria this pregnancy, prior infant with GBS disease, OR unknown status + intrapartum risk factor (preterm < 37 wk, ROM ≥ 18 h, intrapartum temp ≥ 38 °C, or positive intrapartum NAAT). Decisions made by your obstetric team.
What does this mean?
Group B Streptococcus colonises 10–30 % of pregnant women, usually harmlessly. Without intrapartum antibiotic prophylaxis (IAP), about 1–2 % of babies born to colonised mothers develop early-onset GBS disease (sepsis, pneumonia, meningitis — case fatality ~5 %). Universal screening at 36+0–37+6 weeks plus IAP for positive carriers has reduced US early-onset GBS incidence from ~1.7 to ~0.25 per 1,000 births since the 1990s. The 4-hour rule matters because adequate placental transfer requires that interval — which is why penicillin / ampicillin / cefazolin given ≥ 4 h before delivery are counted as “adequate” in newborn EOS-risk calculators (Kaiser model), while clindamycin / vancomycin are not. Some UK units (RCOG GTG 36) favour a risk-based rather than universal- screening approach — both work if applied consistently.
Introduction
Group B Streptococcus (GBS) is the leading cause of early-onset neonatal sepsis in high-income countries. Intrapartum antibiotic prophylaxis (IAP) given ≥ 4 hours before delivery reduces transmission and early-onset GBS disease by ~80 %.
Indications
- Positive 36+0–37+6 wk vaginal-rectal screen.
- GBS bacteriuria at any concentration this pregnancy.
- Prior infant with invasive GBS disease.
- Unknown status plus: preterm labour, ROM ≥ 18 h, intrapartum temp ≥ 38 °C, or positive intrapartum NAAT.
First-line regimen
- Penicillin G — 5 million units IV load, then 2.5–3 million units IV every 4 h until delivery.
- Alternative: Ampicillin 2 g IV load, then 1 g IV every 4 h.
Penicillin-allergy alternatives
- Mild allergy (rash, no anaphylactic features): Cefazolin 2 g IV load, then 1 g IV q8h.
- Severe allergy + GBS clindamycin-sensitive: Clindamycin 900 mg IV q8h.
- Severe allergy + clindamycin-resistant or untested: Vancomycin 20 mg/kg IV q8h (max 2 g per dose; infusion ≥ 60 min).
What counts as adequate prophylaxis?
For neonatal risk-stratification (Kaiser EOS / AAP), “adequate” means a dose of penicillin, ampicillin, or cefazolin given ≥ 4 hours before delivery. Clindamycin and vancomycin are NOT counted as adequate due to uncertain placental transfer.
How this relates to newborn care
After delivery, the Kaiser EOS calculator uses whether IAP was adequate as a major input, substantially reducing unnecessary newborn work-up and antibiotic exposure (60–70 % reduction per Achten 2019 JAMA Pediatr meta-analysis).
Limitations
- IAP prevents only early-onset (< 7 days) GBS disease, not late-onset (1 week–3 months).
- Universal screening misses late colonisation after 37 weeks.
- Some women refuse IAP — shared decision-making documented.
- The calculator is a check, not a prescription — your team will reconcile with allergy history and local protocol.
Sources
- ACOG Committee Opinion 797. Prevention of Group B Streptococcal Early-Onset Disease in Newborns. 2020 reaffirmed 2024.
- CDC. Group B Strep (GBS) Prevention Guideline. 2024 update.
- RCOG Green-top 36. The Prevention of Early-onset Neonatal Group B Streptococcal Disease. 2017.
- Puopolo KM, Lynfield R, Cummings JJ; AAP Committee on Fetus and Newborn. Management of Infants at Risk for Group B Streptococcal Disease. Pediatrics 2019;144:e20191881.
- Achten NB, et al. Association of use of the neonatal early-onset sepsis calculator with reduction in antibiotic therapy. JAMA Pediatr 2019;173:1032–40.
Frequently asked questions
Why is GBS prophylaxis given?
Group B Streptococcus (Streptococcus agalactiae) colonises the vagina and rectum in 10–30 % of pregnant women. Most never have any problem, but vertical transmission to the baby during labour can cause early-onset GBS disease (sepsis, pneumonia, meningitis) in 1–2 % of babies born to colonised mothers, with ~5 % case-fatality. Intrapartum antibiotic prophylaxis (IAP) with penicillin or an alternative reduces early-onset GBS disease by ~80 %, and US incidence has fallen from ~1.7 to ~0.25 per 1,000 births since universal screening began in the 1990s.
When is GBS prophylaxis indicated?
Per ACOG CO 797 (2020 reaffirmed 2024) and CDC GBS 2024, IAP is indicated for any of: (1) positive 36+0–37+6 wk vaginal-rectal screen; (2) GBS bacteriuria this pregnancy at any concentration; (3) prior infant with invasive GBS disease; or (4) unknown GBS status PLUS any of: preterm labour < 37 weeks, ROM ≥ 18 hours, intrapartum temperature ≥ 38 °C, or positive intrapartum NAAT. Planned caesarean before labour onset with intact membranes does NOT require IAP for GBS even if positive.
What is the first-line antibiotic?
Penicillin G is preferred: 5 million units IV loading dose, then 2.5–3 million units IV every 4 hours until delivery. Ampicillin 2 g IV loading then 1 g IV every 4 hours is an acceptable alternative; some units choose ampicillin when broader coverage is also wanted (e.g. chorioamnionitis). Both are equally effective for GBS.
What if I am allergic to penicillin?
It depends on severity. For a MILD prior reaction (rash without urticaria, anaphylaxis, or respiratory features), cefazolin 2 g IV loading then 1 g IV every 8 hours is used — cefazolin has minimal cross-reactivity with penicillin. For SEVERE allergy (anaphylaxis, urticaria, bronchospasm, or unknown reaction), the GBS isolate should be tested for clindamycin susceptibility: if sensitive, use clindamycin 900 mg IV every 8 hours; if resistant or untested, use vancomycin 20 mg/kg IV every 8 hours (max 2 g per dose, infused over ≥ 60 minutes).
What counts as adequate prophylaxis?
Penicillin, ampicillin, or cefazolin given ≥ 4 hours before delivery is considered ADEQUATE for newborn risk stratification (Kaiser EOS calculator, AAP 2018). Clindamycin and vancomycin are NOT counted as adequate because of uncertain placental transfer pharmacokinetics. This matters because babies born to mothers with adequate prophylaxis can be observed clinically rather than receiving routine work-up and antibiotics — a major reduction in NICU separation and antibiotic exposure.
What is the difference between universal screening and risk-based prophylaxis?
Universal screening (US, most of Europe) tests every pregnant woman at 36+0–37+6 wk and gives IAP to all positives. Risk-based (UK RCOG GTG 36) does NOT routinely screen but gives IAP based on risk factors: prior GBS-affected baby, GBS bacteriuria, preterm labour, ROM ≥ 18 h, intrapartum fever ≥ 38 °C. Both approaches reduce early-onset GBS disease substantially; the choice reflects local epidemiology, cost-effectiveness analyses, and antibiotic stewardship priorities. NICE NG25 recommends offering RCOG-style risk-based assessment.
Does IAP cause harm to the baby?
The benefit (preventing serious early-onset GBS sepsis) clearly outweighs the small risks. Concerns about IAP altering the infant microbiome are real but modest, and ALL antibiotic exposure during labour has similar effects. There is no evidence linking GBS IAP to long-term harm (asthma, allergies, obesity have all been studied). Late-onset GBS disease (1 week–3 months) is NOT prevented by intrapartum IAP — its mechanism is different (often household transmission).
How does this relate to other calculators on BumpBites?
BumpBites has companion tools: /calculators/eos-sepsis applies the Kaiser Permanente neonatal sepsis calculator to babies after delivery (using whether IAP was adequate as one input), reducing unnecessary newborn antibiotics by 60–70 %. /calculators/pediatric-dose covers paediatric antibiotic dosing for the wider context. /calculators/antenatal-steroids covers a different but related preterm-birth intervention.