Group B strep (GBS) culture results typically take 24 to 48 hours, though some positives can be detected in as few as 10 hours. The exact turnaround depends on the type of test your provider orders and how your lab processes the sample. Here’s what to expect at each step.
Standard Culture Results: 24 to 48 Hours
The most common GBS test is a bacterial culture. Your provider swabs your vagina and rectum with a cotton swab, and the sample is placed in a special broth that encourages GBS bacteria to grow. Labs using certain types of broth media can detect positive results directly from the broth in as quickly as 10 hours, with the majority of specimens reported within 24 hours. Negative results take longer to confirm because the lab needs to wait long enough to be confident no bacteria are growing, which is why the full window extends to 48 hours.
Once the sample reaches the lab, it goes into an enrichment broth first, then gets transferred to a culture plate or tested further. That enrichment step is standard across nearly all testing methods and accounts for much of the wait time. If your result is positive, you’ll likely hear back on the faster end. If it’s negative, expect closer to the full two days.
Rapid PCR Testing: About 2 Hours
Some hospitals use a molecular test (PCR) that can identify GBS genetic material much faster than a traditional culture. These tests produce results in roughly one to two hours, making them useful when results are needed during labor itself. In hospitals that use intrapartum PCR testing, results are typically in the patient’s file within two hours of the swab being collected.
PCR testing is not the default at most practices for routine prenatal screening. It’s more commonly reserved for situations where a woman arrives in labor without a recent culture result on file. Even PCR-based workflows still require an enrichment broth step before the molecular analysis, which is why they take one to two hours rather than minutes. Still, this is significantly faster than the 24-to-48-hour window for culture alone.
When the Test Is Done
Current guidelines recommend universal GBS screening between 36 weeks and 37 weeks, 6 days of pregnancy. This timing is intentional: it creates a five-week window in which the culture result remains valid, covering births that happen up to at least 41 weeks. If you deliver within that window, your provider will already have your result on hand.
The screening applies regardless of whether you’re planning a vaginal delivery or a cesarean. The only exceptions are women who already have a clear indication for treatment during labor, such as GBS found in a urine culture during the current pregnancy or a previous baby affected by GBS infection.
What Happens to Your Sample
After collection, the swab needs to reach the lab in reasonable condition. Guidelines recommend transporting swabs in a standard transport medium for up to four days at room or refrigeration temperature. Newer transport systems can preserve GBS for even longer, with full recovery demonstrated after up to six days at room temperature. So a short delay between your appointment and the lab processing your sample won’t compromise the result.
If Results Aren’t Back Before Labor
Sometimes labor starts before your culture result is available. This can happen if you deliver prematurely, if your test was drawn late, or if results are simply delayed. When GBS status is unknown at the time of delivery, your care team makes decisions based on risk factors present during labor, such as fever, prolonged time after your water breaks, or preterm delivery. In those situations, you may receive antibiotics through an IV as a precaution.
For the antibiotics to be most effective, they ideally need to be running for at least four hours before delivery. This gives the medication enough time to reach adequate levels in your bloodstream and cross into the amniotic fluid. If labor is progressing quickly, your team will still start treatment, but the shorter window may mean additional monitoring for your baby afterward.
What Happens After Delivery
If you tested positive for GBS or your status was unknown, your baby will be watched closely in the hours after birth. Most cases of early-onset GBS infection in newborns show symptoms within the first 12 to 24 hours. Data from the CDC’s surveillance network shows that nearly 95% of early-onset GBS cases are diagnosed within 48 hours of birth. Monitoring involves regular physical assessments, checking temperature, breathing, and feeding patterns. The vast majority of babies do perfectly fine, and only a small percentage of those being observed end up needing antibiotics themselves.
If you received a full course of IV antibiotics before delivery and your baby looks well at birth, the observation period is generally less intensive. Hospitals have their own protocols for how frequently these check-ins happen, but the key window your care team is most focused on is those first 24 to 48 hours.