Group B Streptococcus (GBS) is a common bacterium carried by approximately 10 to 30% of healthy pregnant women. While colonization usually causes no symptoms for the mother, transmission during labor and delivery poses a serious risk to the newborn. GBS can cause life-threatening early-onset neonatal diseases, including sepsis, pneumonia, or meningitis. Prevention focuses on identifying colonized mothers during the prenatal period and administering protective measures during labor.
Understanding the GBS Screening Protocol
The current standard of care involves screening all pregnant women for GBS colonization late in the third trimester. Testing is performed between 36 weeks and 0 days and 37 weeks and 6 days of gestation, as this window is highly predictive of the mother’s GBS status at delivery. The screening involves collecting a single swab specimen from both the lower vagina and the anorectum to maximize detection. This universal screening determines which women are GBS carriers and require antibiotics during labor to protect the baby.
The Standard Prevention Strategy: Antibiotic Administration
For a woman whose prenatal screening test is positive for GBS, the standard method of prevention is Intrapartum Antibiotic Prophylaxis (IAP). IAP involves the intravenous administration of antibiotics during labor to reduce the bacterial load and prevent transmission to the infant. The antibiotic of choice for this prophylaxis is Penicillin G, which is highly effective against GBS.
The typical dosing regimen for Penicillin G begins with a loading dose of 5 million units given intravenously. This is followed by a maintenance dose of 2.5 to 3.0 million units administered intravenously every four hours until the delivery occurs. Ampicillin is an acceptable alternative, with a similar dosing schedule of a 2-gram initial dose followed by 1 gram every four hours.
The efficacy of IAP relates directly to the duration of treatment before birth. To be considered adequately treated, the mother must receive the antibiotics for at least four hours prior to delivery. This duration ensures high concentrations of medication reach the amniotic fluid and fetal bloodstream, significantly reducing the risk of neonatal GBS disease.
Prevention Protocols for High-Risk and Unknown Status
Prevention protocols address scenarios where GBS status is unknown or high-risk factors are present. For example, a mother who had a previous infant with invasive GBS disease is automatically given IAP in all subsequent pregnancies, bypassing current screening. Similarly, if GBS is detected in the urine at any point during the current pregnancy, IAP is required during labor.
When GBS status is unknown at the onset of labor, certain clinical risk factors trigger the need for IAP. These conditions increase the likelihood of bacterial transmission, making empirical antibiotic treatment necessary. Risk factors include labor beginning before 37 weeks gestation, a maternal fever of 100.4°F (38.0°C) or higher during labor, or ruptured membranes for 18 hours or longer.
Managing Penicillin Allergy
When a woman reports a penicillin allergy, the choice of antibiotic depends on the severity of her reaction. For a non-severe allergy, the preferred alternative is Cefazolin, administered with a 2-gram loading dose followed by 1 gram every eight hours. If the allergy history suggests a high risk for anaphylaxis, susceptibility testing of the GBS isolate is performed to guide treatment. If the isolate is susceptible to Clindamycin, that drug is used; otherwise, Vancomycin is administered.