At what gestational age should a group B strep (Group B Streptococcus) swab be performed?

Group B Streptococcus Screening Should Be Performed at 36 0/7 to 37 6/7 Weeks of Gestation

Group B Streptococcus (GBS) screening should be performed at 36 0/7 to 37 6/7 weeks of gestation through a vaginal-rectal swab culture to optimize detection of maternal colonization and prevent neonatal GBS disease. 1

Current Recommendations for GBS Screening

The timing of GBS screening has evolved based on evidence showing optimal detection of colonization that persists until delivery:

• Previously, the CDC recommended screening between 35-37 weeks' gestation 2
• Current ACOG guidelines have refined this window to 36 0/7 to 37 6/7 weeks' gestation 1, 3

This updated timing recommendation provides the best sensitivity and specificity for detecting women who will remain colonized at delivery, improving the accuracy of risk assessment for early-onset GBS disease in newborns.

Proper Specimen Collection Technique

The accuracy of GBS screening depends on proper collection technique:

• Collect specimens from both the lower vagina (vaginal introitus) followed by the rectum (through anal sphincter)
• Use the same swab or two different swabs for both sites
• Cervical, perianal, perirectal, or perineal specimens are not acceptable
• A speculum should not be used for culture collection 2
• Specimens should be placed into a nonnutritive transport medium 1

Special Considerations for Preterm Labor

For women presenting with signs and symptoms of preterm labor (<37 weeks):

• Obtain vaginal-rectal swab for GBS culture at hospital admission unless a culture was performed within the preceding 5 weeks
• Start GBS prophylaxis while awaiting culture results
• If not in true labor, discontinue prophylaxis
• If culture results become available and are negative, discontinue prophylaxis 2

Management Based on GBS Status

Women who should receive intrapartum antibiotic prophylaxis include:

• Those with positive GBS screening at 36 0/7 to 37 6/7 weeks
• Those with GBS bacteriuria during the current pregnancy (regardless of colony count)
• Those with a previous infant with invasive GBS disease
• Those with unknown GBS status at labor onset who have risk factors (delivery <37 weeks, membrane rupture ≥18 hours, or intrapartum fever ≥100.4°F/38.0°C) 1, 4

Common Pitfalls to Avoid

1. Incorrect timing of screening: Screening before 36 weeks may miss women who become colonized later in pregnancy, while screening after 38 weeks may not provide results before delivery

2. Improper specimen collection: Failing to sample both vaginal and rectal sites significantly reduces detection rates

3. Relying on previous pregnancy results: GBS status in a previous pregnancy does not predict current status unless there was a previous GBS-affected infant 1

4. Treating with oral antibiotics during pregnancy: This is ineffective in eliminating carriage or preventing neonatal disease 1

5. Delaying necessary obstetric interventions: While 4+ hours of antibiotic prophylaxis is ideal, interventions should not be delayed solely to provide this duration of antibiotics 3

The implementation of universal GBS screening at the appropriate gestational age (36 0/7 to 37 6/7 weeks) has significantly reduced the incidence of early-onset GBS disease in newborns, making this a critical component of prenatal care.