Group B Streptococcus, Perinatal

Group B Streptococcus (GBS, Streptococcus agalactiae) is a gram-positive bacterium that is a common, usually harmless component of the normal lower gastrointestinal and genital tract flora — an estimated 10-30% of pregnant people are colonized in the vagina and/or rectum at any given time, and colonization can be transient, intermittent, or persistent across a single pregnancy.^[1] Colonization on its own is not an infection, causes no maternal symptoms in the great majority of cases, and does not by itself require antepartum treatment (see Non-Pharmacologic Management for why treating colonization before labour does not help).

The entire clinical stakes of this chapter are about the newborn. During labour and delivery — through ascending infection after membrane rupture or passage through a colonized birth canal — GBS can be vertically transmitted to the neonate. Roughly half of neonates born to a GBS-colonized parent become colonized themselves, and in the complete absence of intrapartum prophylaxis, about 1-2% of those exposed newborns go on to develop early-onset GBS disease: sepsis, meningitis, or pneumonia, historically one of the leading infectious causes of neonatal morbidity and mortality.^[2] Universal antenatal screening paired with correctly timed intrapartum antibiotic prophylaxis (IAP) is the single most effective prevention strategy available and has transformed this disease's incidence — from roughly 1.8-4.0 cases per 1,000 live births before universal prevention strategies existed, down to approximately 0.2-0.25 cases per 1,000 live births in the contemporary screening-and-prophylaxis era.^[1]

For a pharmacist, the highest-yield content here is the penicillin-allergy decision tree that determines which IAP agent is correct for a given patient (see Choose Initial Treatment) — a genuinely common, genuinely consequential decision point, since choosing the wrong agent can mean either an avoidable anaphylaxis risk or a prophylaxis regimen that silently fails against a resistant organism.

Colonization, bacteriuria, and invasive disease are three different things

These three terms are often used loosely but describe genuinely different clinical situations, each with a different management implication.

Factors that indicate a need for intrapartum prophylaxis regardless of a negative or unavailable screening culture

• GBS bacteriuria at any colony count during the current pregnancy — automatically classified GBS-positive; no antenatal screening culture is needed to confirm this (see Assessment & Diagnosis).
• A previous infant affected by invasive GBS disease — automatically classified GBS-positive in this pregnancy regardless of any current culture result.
• Unknown GBS status at the onset of labour (no culture done, culture result unavailable, or culture collected more than 5 weeks before delivery — see Assessment & Diagnosis) together with any of the following:
  • Preterm labour or delivery before 37 weeks' gestation.
  • Rupture of membranes 18 hours or longer before delivery.
  • Intrapartum fever ≥38°C (100.4°F).

Factors associated with GBS colonization or with early-onset disease

• Young maternal age.
• Multiparity is not protective — colonization status can differ between pregnancies in the same person, which is why every pregnancy is screened independently rather than relying on a previous pregnancy's result (short of the two automatic-positive exceptions above).
• Multiple gestation — same screening and management principles apply, with no unique dosing change for GBS prophylaxis itself (see Special Populations).
• Population-level epidemiologic data show higher early-onset GBS disease rates among Black infants in North America compared with other groups, for reasons not fully explained by identified risk-factor differences alone — a disparity worth being aware of rather than a reason to alter individual screening or prophylaxis practice, which remains universal.^[1]

Universal antenatal screening

Offer every pregnant patient a combined vaginal-rectal swab culture for GBS — one swab passed first through the lower vagina, then through the anal sphincter into the rectum — regardless of planned mode of delivery, including patients planning a cesarean birth (since labour can begin, or membranes can rupture, before a scheduled cesarean occurs).^[3]

Timing: SOGC recommends screening at 35-37 weeks' gestation.^[3] ACOG's more recently updated window is 36 0/7 to 37 6/7 weeks — a small shift specifically designed to extend the culture's validity window to cover births up to 41 0/7 weeks (see below), since the original 35-week starting point left a shorter margin for a patient who delivers at or near term.^[2] Follow local hospital protocol for the exact window used; the key point for pharmacist counselling is that this is a universal, culture-based screening approach, not one restricted to patients with identifiable risk factors — universal screening has been shown to be more sensitive than a risk-factor-based approach at identifying patients who will benefit from IAP.^[1]

Culture validity window: a negative screening culture is considered valid for 5 weeks from collection. A patient who delivers more than 5 weeks after her screening culture is managed as though her GBS status is unknown at labour — this is the practical reason ACOG shifted the recommended screening window later, to keep more patients' cultures valid through a wider range of delivery timing.^[2]

Who does not need a screening culture — automatic GBS-positive classification

Two groups are classified as GBS-positive by history alone, and a 35-37 week screening culture is not needed (and cannot override this classification even if it comes back negative):

• GBS bacteriuria identified at any colony count during the current pregnancy (from a routine prenatal urine culture, regardless of whether it met the ≥10⁵ CFU/mL threshold for antepartum UTI treatment).
• A previous infant affected by invasive GBS disease, in any prior pregnancy.

Susceptibility testing — plan ahead for the penicillin-allergy pathway

For any patient reporting a high-risk penicillin allergy (see Choose Initial Treatment for the risk-stratification criteria), order clindamycin (and erythromycin) susceptibility testing on the GBS isolate at the time the screening culture is collected — including reflex testing for inducible clindamycin resistance (D-zone testing) if the isolate is erythromycin-resistant. Ordering this proactively, rather than waiting until the patient presents in labour, is what makes the clindamycin option actually usable when it is needed (see Choose Initial Treatment).^[1]

Intrapartum status unknown

A patient presenting in labour with genuinely unknown GBS status (no culture performed, result unavailable, or culture more than 5 weeks old) is managed using the risk-factor criteria in Risk Factors — treat as GBS-positive if preterm labour/delivery, rupture of membranes ≥18 hours, or intrapartum fever ≥38°C is present. A rapid intrapartum nucleic acid amplification test (NAAT), where available, can inform this decision in real time for a patient with unknown status and no risk factor, but is not required and does not replace universal antenatal culture-based screening as the primary strategy.^[2]

• Correctly identify GBS-positive status through universal antenatal screening, or through the recognized automatic-positive exceptions (GBS bacteriuria this pregnancy, previous affected infant).
• Prevent early-onset neonatal GBS sepsis, meningitis, and pneumonia through correctly selected, correctly timed intrapartum antibiotic prophylaxis.
• Select the correct IAP agent for every patient, navigating the penicillin-allergy decision tree accurately — avoiding both an avoidable anaphylaxis risk from a first-line beta-lactam and an unnecessarily broad-spectrum agent (clindamycin without confirmed susceptibility, or vancomycin) when a narrower option is genuinely safe.
• Achieve adequate antibiotic exposure (≥4 hours before delivery) whenever feasible, without inappropriately delaying an otherwise medically indicated delivery to "complete" that window.
• Recognize and treat intrapartum chorioamnionitis with appropriately broad-spectrum antibiotics, distinct from GBS-specific prophylaxis.
• Ensure appropriate postnatal observation of the newborn, matched to maternal risk factors and the adequacy of intrapartum prophylaxis actually received.
• Avoid antepartum antibiotic treatment of asymptomatic GBS colonization outside of labour, which does not reduce neonatal risk and unnecessarily promotes antimicrobial resistance.

• There is no effective non-pharmacologic strategy for preventing GBS transmission to the newborn — correctly timed IAP at labour is the only intervention shown to reduce early-onset disease risk.
• Intravaginal chlorhexidine washing during labour has been studied specifically as a possible alternative or adjunct to IAP and has not been shown to reduce the incidence of neonatal GBS disease — it is not recommended as a substitute for antibiotic prophylaxis.^[1]
• Oral or vaginal probiotics to reduce antenatal GBS colonization have insufficient evidence of efficacy and are not recommended as a substitute for IAP.
• Antepartum antibiotic treatment of an asymptomatic positive screening culture, before labour, is not recommended — recolonization after antepartum treatment is common, so treating the culture result itself (rather than treating at labour) does not reliably reduce neonatal transmission risk and unnecessarily contributes to antimicrobial resistance pressure.
• Mode of delivery should not be altered because of GBS-positive status alone — a cesarean delivery is not offered, and does not need to be offered, purely to avoid GBS transmission; IAP indications and exceptions are as described in Choose Initial Treatment regardless of the planned delivery route.
• Standard newborn care — delayed cord clamping, skin-to-skin contact, and routine feeding support — is not altered by maternal GBS status alone in a well-appearing infant with adequate maternal IAP.
• Patient counselling at the time of screening (ideally well before labour) should explain that a positive result means colonization, not an infection needing treatment right now, and should set expectations for what happens at hospital admission (an IV antibiotic started once labour begins or membranes rupture) — this avoids confusion or alarm when the plan is explained again at triage.

Preterm labour and PPROM with unknown GBS status

A patient presenting in threatened or actual preterm labour, or with preterm premature rupture of membranes (PPROM), whose GBS status is unknown should have a screening culture collected at admission and be started on IAP empirically while that result (or the clinical course) is pending — preterm delivery is itself one of the risk-factor criteria that triggers empiric treatment for unknown status (see Risk Factors). IAP for GBS is a distinct consideration from the broader-spectrum latency antibiotics sometimes used to prolong pregnancy after PPROM; both may be indicated concurrently per institutional protocol.^[3]

Planned cesarean delivery

As noted in Choose Initial Treatment, IAP is not indicated for a GBS-positive patient undergoing a genuinely pre-labour, intact-membrane planned cesarean — but universal screening is still offered to this group, since labour or rupture of membranes can occur before the scheduled procedure and would then trigger standard IAP indications.^[3]

Multiple gestation

The same screening timing, IAP indications, and dosing apply — there is no unique dose adjustment for GBS prophylaxis specifically in a twin or higher-order pregnancy.

Renal impairment

Vancomycin dosing may need adjustment for significant renal impairment; consult institutional protocol or pharmacy for weight- and renal-function-based dosing in this situation rather than using the standard interval unmodified.

Obesity

Some institutional protocols use a higher vancomycin loading approach in patients with a significantly elevated body mass index; follow local protocol.

Breastfeeding

Penicillin G, ampicillin, cefazolin, clindamycin, and vancomycin are all considered compatible with breastfeeding.

Neonatal implications of inadequately prophylaxed exposure

Infants born to a GBS-positive (or risk-factor-treated) parent who did not receive adequate IAP — defined as penicillin G, ampicillin, or cefazolin given 4 or more hours before delivery — require closer clinical observation after birth per current well-baby protocols, even when the infant appears entirely well at delivery (see Monitoring & Follow-Up for the specific observation pathway). This is a genuinely common real-world scenario — labour that progresses faster than the 4-hour window allows — and does not by itself mean something went wrong; it means the newborn care plan needs to reflect it.^[4]

Neonatal observation, stratified by maternal IAP adequacy and infant appearance

Current guidance moves away from automatic blood work and empiric antibiotics for every GBS-exposed newborn, toward a more individualized approach based on the infant's clinical appearance and the adequacy of maternal prophylaxis actually received.^[4]

• Ill-appearing infant, at any point: full diagnostic evaluation (blood culture, CBC with differential, and consideration of lumbar puncture and chest x-ray if respiratory symptoms are present) and immediate empiric antibiotic therapy, regardless of maternal GBS status or IAP history.
• Well-appearing infant, but the mother had signs of chorioamnionitis: blood culture ± CBC with differential, with empiric antibiotics started pending results.
• Well-appearing infant, no maternal chorioamnionitis, IAP adequate (penicillin G, ampicillin, or cefazolin ≥4 hours before delivery): routine newborn care and standard observation — no additional GBS-specific testing or intervention is required.
• Well-appearing infant, no maternal chorioamnionitis, IAP inadequate or not given despite an indication: serial vital signs and clinical examinations for 36-48 hours before discharge, rather than automatic blood culture and empiric antibiotics for every such infant. Some institutions instead use a validated multivariate neonatal early-onset sepsis risk calculator to individualize this decision — follow local protocol for which approach is in use.^[4]

Maternal postpartum monitoring

Watch for signs of postpartum endometritis (fever, uterine tenderness, foul-smelling lochia), particularly after prolonged labour or prolonged rupture of membranes.

Handoff to the neonatal/pediatric team

Clearly document and communicate maternal GBS status, the specific IAP agent given, the time it was started relative to delivery, and whether the 4-hour adequacy threshold was met — this information directly drives the neonatal observation pathway above, and an incomplete handoff is a genuine, avoidable patient-safety gap.

1. Labour progresses faster than the 4-hour window allows: do not delay delivery to "complete" adequate prophylaxis — continue with whatever IAP has been given, deliver as clinically indicated, and clearly flag inadequate timing to the neonatal team for the appropriate observation pathway (see Monitoring & Follow-Up).
2. A penicillin allergy is disclosed, or reclassified, after an agent has already been started (e.g., a high-risk allergy is discovered after cefazolin has already been given, or a patient recalls anaphylaxis after clindamycin was started without a susceptibility result on file): reassess against the allergy decision tree immediately and switch to the correct regimen as soon as this is recognized — do not simply continue an agent chosen under incomplete information.
3. Clindamycin susceptibility testing was ordered antenatally but the result is not available at the time of labour: treat susceptibility as unknown and use vancomycin — do not assume susceptibility in the absence of a confirmed result.
4. Signs of chorioamnionitis develop during labour despite GBS-specific IAP already running: add broader-spectrum coverage (typically ampicillin plus gentamicin) — chorioamnionitis is a distinct diagnosis that a GBS-only regimen does not adequately treat (see Red Flags & Refer).
5. The neonate develops signs of early-onset sepsis despite documented adequate maternal IAP: IAP substantially reduces but does not eliminate risk — evaluate and treat the newborn per the standard early-onset sepsis pathway regardless of a well-documented, adequately timed maternal regimen.
6. The preferred agent is unavailable or contraindicated for a non-allergy reason (e.g., a supply issue): escalate to pharmacy and the obstetric team promptly to identify the next-best appropriate agent rather than withholding IAP altogether.

• Treating a positive antenatal screening culture with oral antibiotics before labour, in the belief this "clears" the GBS — recolonization is common, and this does not reliably reduce neonatal transmission risk; the only effective intervention is correctly timed IAP at labour.
• Withholding IAP from a GBS-positive patient undergoing induced labour, under the mistaken belief that only spontaneous labour requires prophylaxis — any labour or rupture of membranes in a GBS-positive patient needs IAP, whether induced or spontaneous.
• Giving IAP to a GBS-positive patient undergoing a genuinely pre-labour, intact-membrane planned cesarean delivery — not indicated in this specific scenario, and unnecessarily exposes the patient to antibiotics.
• Classifying a mild, remote, or vague reaction as a "penicillin allergy" requiring cefazolin, clindamycin, or vancomycin without taking a real allergy history — over-classifying low-risk reactions as high-risk pushes patients toward broader-spectrum agents unnecessarily.
• Using clindamycin empirically for a high-risk penicillin allergy without a confirmed susceptibility result — resistance is common enough that this risks a silent prophylaxis failure.
• Treating GBS bacteriuria found on a routine prenatal urine culture as fully resolved with the standard antepartum UTI antibiotic course, and forgetting that this same patient still needs intrapartum GBS prophylaxis at labour regardless of whether the bacteriuria was successfully eradicated.
• Ordering (or requiring) a 35-37 week screening culture in a patient who already has a previous infant with invasive GBS disease — she is already GBS-positive by definition, and a culture (even a negative one) cannot override that classification.
• Treating a negative screening culture as permanently reassuring — cultures are valid for only 5 weeks, and a patient who develops a genuinely new risk factor (preterm labour, prolonged rupture of membranes, intrapartum fever) after a negative or expired culture still needs individualized reassessment.
• Failing to clearly document and hand off maternal GBS status and IAP adequacy to the neonatal/pediatric team — this information directly drives the newborn observation plan.
• Implying to patients that IAP protects against GBS disease broadly — it specifically prevents early-onset disease; it does not prevent late-onset disease occurring after the first week of life.

How to use this table: doses are adult intrapartum obstetric doses for GBS prophylaxis specifically and require patient-specific confirmation (renal function, weight, true allergy history). Drug pricing is intentionally omitted. Select any drug name to open its complete Pharmasist Drug Database page.

First-Line Prophylaxis — No Penicillin Allergy

Penicillin G is preferred given its narrow spectrum; ampicillin is an acceptable alternative.^[1]

Penicillin-Allergic — Low-Risk Allergy

An isolated maculopapular rash, or a distant/vague history without an immediate hypersensitivity feature, is classified low-risk.^[2]

Penicillin-Allergic — High-Risk Allergy (Anaphylaxis, Angioedema, Respiratory Distress, or Immediate Urticaria)

Agent selection depends entirely on antenatal clindamycin susceptibility testing (see Assessment & Diagnosis, where this is ordered proactively).^[1]

A different diagnosis, not GBS-specific prophylaxis

Evidence note: drug inventory, dosing, and comments reflect the current SOGC guideline, cross-checked against the CDC and ACOG guidance it draws from for exact dosing intervals.^[3][1][2]