Lucan · Conditions

Group B streptococcus perinatal prophylaxis

Australian guidance: RANZCOG C-Obs 19 GBS Screening (2019)

1. Definition and epidemiology

Group B streptococcus (GBS), or Streptococcus agalactiae, is a gram-positive encapsulated bacterium that colonises the lower genital and gastrointestinal tracts of 10 to 30% of pregnant women in Australia. Without intrapartum antibiotic prophylaxis (IAP), the incidence of early onset GBS sepsis (EOGBS) in neonates is estimated at 0.4 to 4 per 1000 live births in Australia, with a contemporary case fatality rate of approximately 14%. Preterm infants face a mortality rate approximately ten times higher than term infants when infected, making prevention in high-risk pregnancies a clinical priority.

2. Pathophysiology

GBS is a beta-haemolytic streptococcus that elaborates several virulence factors relevant to neonatal disease. The polysaccharide capsule of serotypes Ia, Ib, II, III, and V impairs opsonophagocytosis by neonatal neutrophils, which have immature complement activation and low opsonic antibody levels. During passage through the birth canal, a colonised neonate aspirates or ingests GBS from vaginal secretions, allowing mucosal invasion via pili-mediated adherence to respiratory epithelium. Beta-haemolysin and CAMP factor disrupt host cell membranes, facilitating bacteraemia. Early onset disease presents within the first seven days of life, most commonly as neonatal pneumonia and respiratory distress from haematogenous seeding of the lungs. Late onset disease (one week to three months) typically presents as meningitis or septicaemia via a different pathogenesis, and is not prevented by intrapartum antibiotics because it reflects postnatal acquisition rather than vertical transmission during labour.

3. Clinical presentation and diagnostic criteria

Maternal GBS colonisation is entirely asymptomatic in the vast majority of women and does not constitute infection. Clinical concern arises when maternal risk factors signal increased risk of vertical transmission and EOGBS. Risk factors established by RANZCOG C-Obs 19 include spontaneous onset of labour before 37 weeks gestation, rupture of membranes for 18 hours or longer, maternal fever of 38 degrees Celsius or above, a previous infant with EOGBS or late onset GBS, GBS bacteriuria detected during the current pregnancy, known GBS carriage confirmed by culture in the current pregnancy, clinical chorioamnionitis, and an existing twin with current EOGBS. Neonatal EOGBS presents within hours of birth with respiratory distress, apnoea, hypoxia, temperature instability, and circulatory compromise progressing rapidly to septic shock and multi-organ failure if untreated.

4. Investigations and AU-specific cutoffs

Two complementary strategies are endorsed by RANZCOG for identifying women who should receive IAP. Universal culture-based screening collects a combined low vaginal and anorectal swab at 35 to 37 weeks gestation, or 3 to 5 weeks before anticipated delivery in women expected to deliver preterm (such as those with poorly controlled diabetes or multiple pregnancy). Swabs must be incubated in selective enriched media (not standard culture conditions) to achieve acceptable sensitivity; failure to specify GBS screening on the pathology request form risks processing as routine culture with false negative rates up to 50%. The negative predictive value of a GBS culture performed within five weeks of delivery is 95 to 98%. A rectovaginal culture at 36 weeks has sensitivity of 91% and specificity of 88.9% for intrapartum colonisation. The alternative strategy is a clinical risk factor-based approach at the time of labour onset without prior screening culture. If carriage status is unknown at labour onset, clinical risk factor-based treatment is the appropriate default. GBS bacteriuria at any titre during pregnancy is a surrogate marker of heavy colonisation and mandates IAP regardless of screening culture result.

5. Management

All maternity services in Australia should have an established prevention plan for EOGBS per Recommendation 1 of RANZCOG C-Obs 19. IAP with intravenous penicillin-G or ampicillin is offered to all women at increased risk of EOGBS (Recommendation 4). Penicillin is preferred over ampicillin because of its narrower spectrum of activity, which reduces selection pressure for resistant organisms. Optimal IAP requires administration at least four hours before delivery to achieve adequate neonatal serum levels; this timing threshold is the key determinant of prophylaxis efficacy. Antepartum oral antibiotics for known GBS colonisation are not effective because they result in a 67% recurrence of colonisation by the time of labour.

Specific clinical scenarios require tailored approaches. For elective caesarean section performed before labour onset and before membrane rupture, IAP is not required irrespective of carriage status. However, if a GBS-positive woman commences labour or has spontaneous rupture of membranes before her planned caesarean section, IAP must be commenced immediately. Women presenting with threatened preterm labour should have a rectovaginal swab taken and IAP commenced if labour establishes, then ceased if labour does not progress. For term pre-labour rupture of membranes in a GBS-positive woman, induction of labour and IAP should be offered without delay. For preterm pre-labour rupture of membranes, a rectovaginal swab is taken and latency antibiotics given independently; if culture subsequently returns positive, IAP is commenced with onset of labour. At 34 weeks or more with confirmed GBS and preterm pre-labour rupture of membranes, immediate delivery is offered. In the setting of clinical chorioamnionitis, broader-spectrum cover is required beyond penicillin alone to treat non-GBS causes of chorioamnionitis. Water birth and water immersion are not contraindicated in GBS carriers who agree to receive IAP. Women who decline IAP must be counselled that their neonate carries a substantially higher EOGBS risk and should be monitored closely for 12 hours after birth.

6. Pharmacology and AU PBS notes

First-line IAP is benzylpenicillin (penicillin-G) given intravenously. The dose follows institutional or jurisdictional protocols, with the critical criterion being initiation at least four hours before delivery. Benzylpenicillin is available on the PBS for inpatient hospital use. Ampicillin is an acceptable alternative with equivalent efficacy against GBS but broader spectrum activity.

For women with known penicillin allergy, antibiotic sensitivities must be specifically requested at the time of GBS culture because approximately 20% of GBS isolates are resistant to clindamycin and 30% to erythromycin. Women who have not experienced anaphylaxis, angioedema, urticaria, or respiratory distress after a penicillin or cephalosporin may receive intravenous cefazolin (first-generation cephalosporin with reliable GBS activity). For women at risk of anaphylaxis with penicillin, the choice between clindamycin and vancomycin depends on GBS sensitivity results. Clindamycin is used if susceptibility is confirmed. Vancomycin is reserved for women at high anaphylaxis risk whose isolate is resistant to clindamycin or erythromycin; the dose is 20 mg per kilogram intravenously every eight hours to a maximum individual dose of 2 grams. Vancomycin is available on the PBS under restricted benefit for documented penicillin anaphylaxis.

7. Complications and monitoring

The primary preventable complication is EOGBS in the neonate, which carries approximately 14% case fatality and a ten-fold higher mortality in preterm infants compared with term infants. Neonates born to mothers who received IAP less than four hours before delivery or who declined IAP require close observation for a minimum of 12 hours after birth, with surveillance for respiratory distress, temperature instability, poor feeding, and haemodynamic compromise. IAP does not prevent late onset GBS disease (one week to three months), which presents predominantly as meningitis or septicaemia; parents should be counselled accordingly. Maternal complications of untreated intrapartum GBS bacteraemia include chorioamnionitis and puerperal sepsis. The risk of maternal anaphylaxis from benzylpenicillin is estimated at 4 per 10,000 to 4 per 100,000, and any morbidity from this reaction is substantially offset by reduction in EOGBS incidence at the population level. Women with a previous GBS-positive infant face approximately 50% risk of GBS colonisation recurrence in a subsequent pregnancy and should be offered rescreening in that pregnancy rather than presumptive IAP without current culture confirmation.

Written for Australian practice and grounded in the guidance named above. Study material only — not medical advice. Always check against current guidelines.