Group B streptococcus in labour — structured oral station (12 minutes)

Opening prompt — "What is the indication for intrapartum antibiotic prophylaxis in this woman?"

Model response, in this order: ^[1]

• "The indication is her positive GBS vaginal-rectal culture at 36 weeks in a woman in established labour. The Cochrane review by Ohlsson confirms that intrapartum antibiotic prophylaxis for known maternal GBS colonisation reduces the incidence of early-onset GBS infection in the neonate."^[1][2]
• "The first-line agent is intravenous benzylpenicillin, with the standard ANZ regimen of 3 grams loading then 1.5 grams every 4 hours until delivery."^[1][4]
• "She has no penicillin allergy, so benzylpenicillin is the appropriate agent. I would start the loading dose as soon as possible to allow the 4-hour window before delivery."^[1]

Examiner is listening for: the indication stated, the first-line agent named, and the dose with the loading dose and the maintenance interval. ^[1]

Probe 1 — "Why is the 4-hour window so important?"

• "Intrapartum antibiotic prophylaxis is considered adequate only when the antibiotic has been given at least 4 hours before delivery. This is the time required to achieve adequate fetal serum and amniotic fluid concentrations of benzylpenicillin above the minimum inhibitory concentration of GBS."^[1][4]
• "Antibiotics given less than 4 hours before delivery are considered inadequate, and the neonatal team needs to know, because the neonatal observation and investigation pathway differs."^[1]
• "The Deegan systematic review (2026) highlighted the limited high-quality evidence supporting current dosing regimens in achieving adequate fetal concentrations, but the 4-hour rule remains the standard clinical threshold."^[4]

Probe 2 — "How does the intrapartum PCR approach differ from universal culture?"

• "Universal culture at 36 to 37 weeks identifies women with GBS colonisation at that time, with a negative culture valid for 5 weeks. This is the approach recommended by ACOG Committee Opinion 797."^[1]
• "Intrapartum PCR uses a rapid nucleic acid amplification test on a vaginal-rectal swab taken at the time of admission to labour, with a result available within 1 to 2 hours. The El Helali studies confirmed equivalent sensitivity to culture when an enrichment step is included."^[7]
• "The intrapartum PCR approach is particularly useful for women with unknown GBS status presenting in labour. It allows targeted IAP based on the current colonisation status, and it is being increasingly used in selected centres."^[7]

Probe 3 — "What is the role of the GBS vaccine?"

• "The maternal GBS vaccine is in late-stage development. The goal is to prevent both early-onset and late-onset GBS disease through transplacental antibody transfer."^[3][8]
• "The capsular polysaccharide conjugate vaccine is the leading candidate, and the Seale 2017 global burden estimates provided the foundational data for the vaccine effort — invasive GBS disease attributable stillbirths and neonatal deaths, and the neurodevelopmental impairment in survivors."^[3][5]
• "If a maternal vaccine were available, it would offer several advantages over the current culture-and-IAP approach: it would prevent late-onset disease, it would not require intrapartum antibiotic administration, and it would reduce the burden of broad-spectrum antibiotic exposure in the neonatal period."^[3][8]

Probe 4 — "What if she had reported a penicillin allergy?"

• "I would first assess the nature of the allergy — a low-risk history such as a mild rash without systemic symptoms allows the use of cefazolin, a first-generation cephalosporin."^[1]
• "A high-risk history such as anaphylaxis requires a different agent. If the GBS isolate is known to be susceptible to clindamycin, I would use intravenous clindamycin 900 mg every 8 hours."^[1]
• "If the isolate is non-susceptible to clindamycin, the only pharmacokinetically and microbiologically validated option is intravenous vancomycin 20 mg per kilogram every 8 hours, with a maximum of 2 grams per single dose, and the dose based on weight and baseline renal function."^[1]
• "Erythromycin is no longer recommended for GBS prophylaxis because of rising resistance."^[1]

Probe 5 — "What if the labour is prolonged and she has not delivered within 8 hours?"

• "I would continue the benzylpenicillin at 1.5 grams every 4 hours throughout the labour, ensuring that she has at least one further dose after the loading dose before delivery."^[1]
• "If she has received benzylpenicillin for over 4 hours before delivery, the IAP is considered adequate. If she has not, the IAP is inadequate, and the neonatal team must be informed at birth."^[1]
• "There is no maximum duration for the benzylpenicillin regimen — it continues until delivery. The 4-hour dosing interval is based on the pharmacokinetics of benzylpenicillin in achieving adequate amniotic fluid concentrations."^[1][4]

Probe 6 — "What is the neonatal management if the IAP is adequate?"

• "If the IAP has been given at least 4 hours before delivery and the neonate is clinically well at birth, the neonate is observed on the postnatal ward with routine vital signs and feeding assessment, with no routine investigation and no empiric antibiotics."^[1][6]
• "Parental advice includes watching for signs of early-onset sepsis: respiratory distress, temperature instability, poor feeding, lethargy. The neonatal team reviews the neonate at the routine examination."^[1]
• "If the neonate develops signs of sepsis at any point, a full septic screen and empiric antibiotics are initiated."^[1]

Probe 7 — "Tell the woman what is happening, in language she will understand."

This is scored. Demonstrate it out loud rather than describing it. ^[1]

• Sit down, at her eye level, with her partner or support person present if she wishes.
• Plain words: "We tested you a few weeks ago for a bacteria called group B strep, which is common and harmless in adults but can cause problems for the baby as it is born. The test came back positive, so we are going to give you an antibiotic through the drip in your arm now to protect the baby."
• Stop and acknowledge: "The antibiotic works best when it has been in your system for at least 4 hours before the baby is born, so we are starting it now to make sure it has time to work."
• The plan: "You will have a dose every 4 hours through the labour. We will let the paediatric team know what we have given you so they can check the baby when it is born. Most babies whose mothers have had this antibiotic do well and do not need any other treatment."
• Outcome honesty: "If for some reason the antibiotic has not had 4 hours to work by time baby is born, the paediatric team will check the baby more closely and may do some tests, just to be safe."^[1]

Probe 8 — "What is the role of the previous infant with invasive GBS disease in future pregnancies?"

• "A previous infant with invasive GBS disease is an absolute indication for IAP in subsequent pregnancies, regardless of the current culture status."^[1]
• "The reason is that the woman is at substantially elevated risk of having another affected infant, even if her current culture is negative. The recommendation applies to all future pregnancies."^[1]
• "This is one of the two indications (the other being GBS bacteriuria in the current pregnancy) that bypass the culture result entirely."^[1]