Group B Streptococcus (GBS) is a common, usually harmless bacterium that can be passed from parent to newborn during labor, occasionally causing serious infection. This calculator walks through the CDC/ACOG decision points that determine whether intrapartum antibiotic prophylaxis is indicated, based on GBS culture status and a small set of intrapartum risk factors. It exists to help you understand and organize the relevant facts before a conversation with the delivery team — not to replace that conversation.
How the algorithm works
The CDC recommends universal vaginal-rectal culture screening for GBS at 36 0/7 to 37 6/7 weeks of gestation (the 2019 update moved this window slightly later than the original 2010 guideline's 35–37 weeks). That culture result drives most decisions: a positive result is a direct indication for intrapartum prophylaxis, and a negative result within 5 weeks of delivery is considered reassuring on its own.
Two situations override the culture result entirely, in either direction. A prior infant with invasive GBS disease, or GBS bacteriuria (any colony count) found during this pregnancy, are absolute indications for prophylaxis even if today's culture is negative. Conversely, a planned cesarean delivery performed before labor begins with membranes intact does not need prophylaxis regardless of GBS status, because the exposure being prevented — intrapartum, vaginal transmission — does not occur in that scenario.
When GBS status was never determined (culture unknown or not done), the decision falls back to three intrapartum risk factors: labor starting before 37 weeks, rupture of membranes 18 hours or longer, or an intrapartum temperature of 100.4°F (38.0°C) or higher. Any one of these present with unknown status is enough to recommend risk-based prophylaxis.
Reading your result
The result names the single factor that determined the recommendation — for example a positive culture, a prior affected infant, or a specific risk factor combined with unknown status — so you can see exactly which input is doing the work. The Risk Factor Checklist tab lists every factor the algorithm considers and whether your current inputs satisfy it, which is useful for reviewing the full picture rather than just the final answer.
This output is a risk/eligibility summary meant to support a conversation with your delivery team, not an antibiotic order. It does not address antibiotic selection or penicillin-allergy management — those are separate clinical decisions made at the bedside based on allergy history and, where relevant, sensitivity testing.
Limits and how it should be used
This tool implements a simplified version of the CDC/ACOG algorithm for common scenarios. It does not cover every edge case in the full guideline — for example, GBS status from a prior pregnancy, incomplete or borderline culture results, or nuanced antibiotic-selection guidance for penicillin allergy with varying anaphylaxis risk. Actual practice also varies by institutional protocol.
This calculator is for education and clinical reference only. Decisions about intrapartum antibiotic prophylaxis and about neonatal sepsis risk assessment after delivery must be made by the delivering clinician and pediatric team based on the complete clinical picture, current CDC/ACOG guidance, and local protocol — never by this tool alone.