Group B Strep (GBS) culture results typically take 24 to 48 hours to process in the lab, though it may take a few additional days before your provider shares the result with you. The swab itself is quick and painless, but the waiting period depends on whether your provider’s lab uses a standard culture or a newer rapid test.
Standard Culture: 24 to 48 Hours
The standard GBS screening uses a vaginal and rectal swab that’s sent to a lab and placed in a growth medium. The lab watches to see whether GBS bacteria multiply. In a large multi-center study, the average time for cultures to turn positive was about 15 hours, and GBS tends to grow faster than many other bacteria. By 24 hours, 91% of positive cultures had shown growth. By 36 hours that number climbed to 96%, and by 48 hours it reached 99%.
So the lab work itself is usually done within two days. But that doesn’t always mean you’ll hear back that fast. Many practices batch their lab pickups, and your result still needs to be reviewed and entered into your chart. In practice, most people get their GBS result within 3 to 5 days of the swab. You’ll typically see it pop up on your patient portal, or your provider will share it at your next prenatal visit. If you haven’t heard anything after a week, it’s reasonable to call and ask.
Rapid Tests: Under an Hour
Some hospitals use a rapid molecular test that can detect GBS DNA in under an hour. One widely used version, the Xpert Xpress GBS, requires about one minute of hands-on setup and delivers a result in roughly 43 minutes. These tests aren’t used for routine prenatal screening. They’re reserved for situations where a quick answer is needed, most commonly when someone arrives in labor without a GBS result on file. If you go into labor early or your previous result has expired, this is the test your hospital would likely run.
When Screening Happens and How Long Results Stay Valid
GBS screening is recommended during the 36th or 37th week of pregnancy. The timing is deliberate: GBS bacteria come and go in your body naturally, so a result from earlier in pregnancy wouldn’t reliably predict your status at delivery. Testing at 36 to 37 weeks creates a five-week window of valid results, covering births up to at least 41 weeks.
If you deliver after that window closes, or if you go into preterm labor before you’ve been screened, your GBS status is considered unknown. In that case, your care team will make decisions about antibiotics based on other risk factors rather than waiting on a culture.
What a Positive Result Means for Delivery
About 25% of pregnant women carry GBS, and a positive result doesn’t mean you or your baby are sick. It simply means the bacteria are present, and your delivery plan will include IV antibiotics to prevent passing GBS to your baby during birth. The antibiotics work best when given at least four hours before delivery, which is why your provider wants the result well before labor begins.
The goal is straightforward: antibiotics given at least four hours before birth are highly effective at preventing the baby from developing an early GBS infection. This is the main reason the screening timeline, the validity window, and the backup rapid test all exist. Every part of the process is designed to make sure your care team knows your GBS status before you deliver and has enough time to act on it.
If Your Results Aren’t Back Yet
If you’re approaching your due date and still don’t have a result, check your patient portal first. Lab results sometimes land there before your provider has called. If nothing is posted, call your provider’s office and ask them to look it up directly. A missing or delayed result isn’t dangerous on its own, but having it on file before labor starts avoids the need for rapid testing or precautionary antibiotics at the hospital.
If labor begins before your result comes back, your hospital can run a rapid test on-site. You won’t be left without a plan. But getting the standard culture done on time, during weeks 36 to 37, gives everyone the most flexibility and the clearest path forward.