The Group B Strep (GBS) test is done between 36 weeks 0 days and 37 weeks 6 days of pregnancy. This screening window, recommended by the American College of Obstetricians and Gynecologists (ACOG), applies to all pregnant women regardless of how they plan to deliver.
Why the Test Happens at 36 to 37 Weeks
GBS is a type of bacteria that naturally lives in the intestines and genital tract. It’s not a sexually transmitted infection and it doesn’t mean anything is wrong. Roughly 1 in 5 pregnant women carry GBS at any given time, though global estimates range from 11% to 35% depending on the region. Carrying the bacteria is called “colonization,” and it typically causes no symptoms in adults.
The reason screening happens so close to the due date is that GBS colonization can come and go. A test done at 20 weeks wouldn’t reliably predict your status at delivery. Testing within five weeks of your due date gives the most accurate picture of whether the bacteria will be present when your baby passes through the birth canal.
What the Test Involves
The test itself is quick and painless. Your provider uses a sterile swab (similar to a cotton swab) to collect a sample from the vagina and the rectum. Both sites are swabbed because GBS commonly lives in the lower intestinal tract and can easily spread to the vaginal area. The sample is sent to a lab for culture, and results typically come back within 24 to 48 hours.
Some providers may offer you the option to collect the swab yourself during the appointment, which studies have shown is just as accurate as provider-collected samples.
What Happens if You Test Positive
A positive result means you’re carrying GBS at the time of the test. It does not mean your baby will definitely get sick, but it does change how your labor is managed. You’ll receive antibiotics through an IV during labor, ideally starting at least four hours before delivery. This timing matters: when antibiotics are given at least four hours before birth, they are highly effective at preventing the bacteria from passing to the baby.
The standard antibiotic is penicillin. If you have a penicillin allergy, your provider will choose an alternative based on the severity of your allergy and lab sensitivity testing. The antibiotics are given every four hours during active labor until delivery, so if you have a longer labor, you’ll receive multiple doses.
Why GBS Matters for the Baby
Without treatment, 40% to 75% of babies born to GBS-positive mothers will pick up the bacteria during delivery. Of those, about 12% will develop early-onset invasive GBS disease, which appears within the first week of life. In newborns, GBS is a leading cause of sepsis (bloodstream infection), pneumonia, and bacterial meningitis.
The stakes with meningitis are particularly serious. About 3 in 10 babies who recover from GBS meningitis may have lasting effects like hearing loss, seizures, cerebral palsy, or delays in learning to move or speak. Globally, GBS causes an estimated 390,000 infant infections each year. This is exactly why universal screening became standard practice: identifying carriers and treating them during labor dramatically reduces these risks.
When the Standard Screening Doesn’t Apply
There are two situations where you’ll automatically receive antibiotics during labor without waiting for the 36-week swab. If GBS was found in your urine at any point during your pregnancy, you’re considered a carrier for the rest of that pregnancy and will be treated during labor. The same applies if you had a previous baby who developed a GBS infection. In both cases, the late-pregnancy swab is unnecessary because the decision to treat has already been made.
For women having a planned cesarean delivery before labor begins and before the water breaks, IV antibiotics for GBS are generally not needed. The bacteria typically pass to the baby during vaginal delivery, so if the baby never enters the birth canal and the membranes are intact, the transmission risk drops significantly. That said, all pregnant women are still screened at 36 to 37 weeks because plans can change: labor might start early, or membranes might rupture before the scheduled surgery.
What if You Deliver Early
If you go into labor before your scheduled screening, your GBS status will be unknown. In this situation, providers consider risk factors like preterm labor (before 37 weeks), prolonged rupture of membranes (water broken for 18 hours or more), or fever during labor. If any of these are present, you’ll typically receive antibiotics as a precaution. Preterm babies are especially vulnerable to GBS infection, so providers tend to err on the side of treatment when the test hasn’t been done yet.
Positive This Pregnancy, Negative Next Time
GBS status can change between pregnancies and even within the same pregnancy. A positive result now doesn’t mean you’ll test positive in a future pregnancy. Each pregnancy requires its own screening at 36 to 37 weeks. Similarly, if you tested negative with a previous baby, you can’t assume you’re still negative this time around. The bacteria fluctuate naturally, which is why the test is repeated every pregnancy and timed as close to delivery as possible.