Residency · Residency · Obstetrics Gynecology
Group B Streptococcus Screening and Intrapartum Prophylaxis
Microbiology and Epidemiology
GBS Overview
Streptococcus agalactiae (Group B Streptococcus, or GBS) is a gram-positive, beta-hemolytic coccus that colonizes the gastrointestinal and genitourinary tracts. The maternal GBS colonization rate ranges from 10 to 30% of pregnant women, and colonization may be transient, intermittent, or chronic. GBS is the leading infectious cause of neonatal morbidity and mortality in the first week of life.
Neonatal GBS Disease
Early-onset disease (EOD) presents within the first 7 days of life, typically within 24 hours, and manifests as sepsis, pneumonia, or meningitis. With universal screening and intrapartum antibiotic prophylaxis (IAP), the incidence has been reduced to approximately 0.2 to 0.3 per 1,000 live births, compared to 1 to 2 per 1,000 without prophylaxis. The case fatality rate is 2 to 3% in term infants but rises dramatically to 20 to 30% in preterm infants.
Late-onset disease (LOD) appears between 7 and 89 days of life. It is not prevented by intrapartum prophylaxis. Meningitis is more common in LOD than in EOD, and the incidence is approximately 0.3 per 1,000 live births.
<image>Diagram illustrating the pathogenesis of neonatal GBS early-onset disease, showing maternal colonization, ascending infection through ruptured or intact membranes, fetal aspiration of infected amniotic fluid, and resulting neonatal sepsis and pneumonia</image>
Screening Protocols
Universal Screening (ACOG/CDC Recommendations)
A vaginal-rectal culture should be obtained at 36 weeks and 0 days to 37 weeks and 6 days of gestation. This timing was updated from 35 to 37 weeks in the 2020 ACOG/CDC revision to optimize predictive value for colonization at delivery. The swab should be collected from the lower vagina (introitus) and then the rectum (through the anal sphincter), without a speculum. Patient self-collection is acceptable and equivalent in sensitivity. The specimen is cultured in selective enrichment broth (Lim broth or equivalent) before subculture. Results remain valid for 5 weeks from collection, and rescreening is necessary if more than 5 weeks elapse before delivery.
Indications for Intrapartum Prophylaxis Without Screening
Certain situations mandate IAP regardless of culture results. GBS bacteriuria at any time during the current pregnancy, at any colony count, is an indication -- these patients do not need a 36 to 37 week vaginal-rectal culture. A previous infant with invasive GBS disease also mandates IAP. When GBS status is unknown at the onset of labor, IAP should be given if any risk factor is present: preterm labor (before 37 weeks), rupture of membranes for 18 hours or more, or intrapartum fever of 38.0 degrees Celsius (100.4 degrees Fahrenheit) or higher.
GBS Bacteriuria
GBS bacteriuria indicates heavy maternal colonization. Any colony count of GBS in urine during pregnancy warrants IAP. Symptomatic urinary tract infections should be treated per standard protocols. Asymptomatic bacteriuria with GBS should be treated per standard ASB guidelines, and IAP should also be provided.
Antibiotic Regimens for Intrapartum Prophylaxis
First-Line
Penicillin G is the preferred agent due to its narrow spectrum: 5 million units IV as a loading dose, followed by 2.5 to 3 million units IV every 4 hours until delivery. Ampicillin is an acceptable alternative at 2 g IV loading followed by 1 g IV every 4 hours, though its broader spectrum may promote resistant organisms.
Penicillin-Allergic Patients
For patients at low risk of anaphylaxis (those with a history of rash without urticaria or angioedema), cefazolin is appropriate at 2 g IV loading followed by 1 g IV every 8 hours. For patients at high risk of anaphylaxis (those with a history of anaphylaxis, angioedema, respiratory distress, or urticaria), susceptibility testing should be requested on the GBS isolate at the time of culture. If the isolate is susceptible to clindamycin, clindamycin 900 mg IV every 8 hours is used. If the isolate is resistant to clindamycin or susceptibility is unknown, vancomycin at 20 mg/kg IV every 8 hours (with a maximum single dose of 2 g) is given. The shift from prior fixed dosing to weight-based dosing reflects the 2020 guideline update.
| Clinical Scenario | Antibiotic | Dosing |
|---|---|---|
| No penicillin allergy (first-line) | Penicillin G | 5 million units IV load, then 2.5-3 million units q4h |
| No penicillin allergy (alternative) | Ampicillin | 2 g IV load, then 1 g q4h |
| Low anaphylaxis risk | Cefazolin | 2 g IV load, then 1 g q8h |
| High anaphylaxis risk, clindamycin-susceptible | Clindamycin | 900 mg IV q8h |
| High anaphylaxis risk, clindamycin-resistant/unknown | Vancomycin | 20 mg/kg IV q8h (max 2 g/dose) |
Timing and Adequacy
IAP is considered adequate when two or more doses of penicillin or ampicillin have been given, or when 4 or more hours of treatment have elapsed before delivery. Cefazolin is also considered adequate at 4 or more hours. For clindamycin and vancomycin, adequacy is less well-established, and neonatal evaluation may be indicated even when treatment has been provided.
<image>Algorithm flowchart for GBS intrapartum antibiotic prophylaxis decision-making, showing branching pathways from GBS culture result through penicillin allergy assessment to specific antibiotic regimen selection</image>
Special Situations
Preterm Labor and PPROM
A GBS culture should be obtained on admission if one has not already been done. IAP should be started if the culture is positive or the status is unknown. If preterm labor resolves and the patient is not delivered, antibiotics should be discontinued. In preterm premature rupture of membranes, GBS cultures should be obtained and IAP provided if the result is positive, the status is unknown, or delivery is imminent.
Planned Cesarean Delivery
GBS prophylaxis is not indicated for planned cesarean delivery performed before the onset of labor with intact membranes, regardless of GBS status. If labor or membrane rupture occurs before the planned cesarean, IAP should be administered per the standard protocol.
Intrapartum Fever
GBS IAP alone is insufficient for suspected intraamniotic infection (chorioamnionitis). If clinical signs of chorioamnionitis develop, the antibiotic regimen should be broadened to ampicillin plus gentamicin.
GBS-Positive Status in Subsequent Pregnancies
GBS status in a prior pregnancy does not predict status in the current pregnancy. Rescreening at 36 to 37 weeks is required in each pregnancy. The exceptions are GBS bacteriuria in the current pregnancy or a prior infant with invasive GBS disease, which mandate IAP without rescreening.
Neonatal Management Considerations
Well-Appearing Newborns
Newborns born to mothers who received adequate IAP should receive routine care with clinical observation for 24 to 48 hours. Those born to mothers with inadequate or no IAP should receive enhanced clinical observation, and depending on risk factors and institutional protocol, a CBC and blood culture may be considered. The AAP/ACOG neonatal GBS guidelines and the neonatal early-onset sepsis calculator are useful tools where utilized.
Symptomatic Newborns
Any newborn showing signs of sepsis -- respiratory distress, temperature instability, lethargy, or poor feeding -- should have an immediate blood culture and CBC drawn and should be started on empiric antibiotics (ampicillin plus gentamicin).
Clinical Pearls
A negative GBS culture at 36 to 37 weeks has a high negative predictive value of 95 to 98%, but colonization can change -- if more than 5 weeks elapse, rescreening is necessary. GBS bacteriuria at any colony count in the current pregnancy is a surrogate marker for heavy genital colonization and mandates IAP. Clinicians should not confuse "adequate IAP" with treatment for chorioamnionitis -- if fever and clinical signs develop, antibiotics must be escalated. Penicillin allergy should be verified and risk-stratified, as true anaphylaxis is rare and cefazolin is safe in most "penicillin-allergic" patients. Weight-based vancomycin dosing (20 mg/kg) replaced the older fixed 1 g dose in the 2020 guidelines. IAP does not prevent late-onset GBS disease, so parents should be counseled on signs of neonatal illness after discharge. Universal screening and IAP have reduced EOD incidence by approximately 80% since implementation in the 1990s.
References
- ACOG Committee Opinion No. 797: Prevention of Group B Streptococcal Early-Onset Disease in Newborns (2020)
- CDC. Prevention of Perinatal Group B Streptococcal Disease: Revised Guidelines. MMWR. 2010;59(RR-10):1-32
- Puopolo KM et al. Management of Infants at Risk for Group B Streptococcal Disease. Pediatrics. 2019;144(2):e20191881
- Verani JR et al. Prevention of perinatal group B streptococcal disease. MMWR Recomm Rep. 2010;59(RR-10):1-36
- ACOG Committee Opinion No. 782: Prevention of Early-Onset Group B Streptococcal Disease in Newborns (reaffirmed 2024)
