The Group B Strep (GBS) test is done during the 36th or 37th week of pregnancy. This timing was updated by the American College of Obstetricians and Gynecologists (ACOG) in 2019, shifting from the previous window of 35 to 37 weeks to ensure results stay accurate closer to your due date.
Why the Test Happens So Late in Pregnancy
GBS is a type of bacteria that naturally lives in the vagina or rectum of about 1 in 4 pregnant women. It’s not a sexually transmitted infection and doesn’t cause symptoms in adults, but it can pass to a baby during delivery and cause serious infections in the first days of life. The bacteria can come and go in your body, which is why testing early in pregnancy wouldn’t be useful. A result from 20 weeks wouldn’t reliably predict whether you’re carrying GBS at the time of birth.
A negative GBS result is considered valid for 5 weeks. The current screening window of 36 to 37 weeks was specifically chosen so that results remain accurate through 41 weeks of gestation, covering even those pregnancies that go past the due date. If more than 5 weeks pass between your test and delivery, you’ll need to be re-screened.
What the Test Involves
The test itself is simple and quick. Your provider uses a swab to collect a sample from your vagina and rectum. Some practices let you do the swab yourself. The sample goes to a lab, where it’s cultured to check for GBS bacteria. Results typically come back within a day or two.
Every pregnancy requires a new test. A positive result in a previous pregnancy doesn’t automatically mean you’ll test positive this time, and a negative result last time doesn’t guarantee you’re clear now.
Who Skips the Swab
There are two situations where the late-pregnancy swab isn’t needed because you’ll automatically receive antibiotics during labor:
- GBS in your urine during this pregnancy. If GBS shows up in a urine culture at any point during your current pregnancy, that’s a marker for heavy colonization. You’ll be treated during labor regardless of a swab result.
- A previous baby with GBS disease. If you’ve had an earlier infant develop an invasive GBS infection, you’re considered high-risk and will receive antibiotics in labor without needing a screening test.
What Happens If You Test Positive
A positive result doesn’t mean anything is wrong with you or your baby right now. It simply means you’ll receive IV antibiotics during labor to prevent the bacteria from passing to your newborn during delivery. The antibiotics need at least 4 hours to work effectively before birth, so your provider will start them as soon as active labor begins or your water breaks.
The standard treatment is a penicillin-type antibiotic. If you have a penicillin allergy, your provider may use a related antibiotic called cefazolin, which is still considered first-line. For people with more severe allergies, alternatives like clindamycin or vancomycin are used instead. In practice, the majority of patients with a documented penicillin allergy end up receiving one of these alternatives, even though current guidelines recommend cefazolin for most of them.
Without antibiotics during labor, GBS-positive mothers have roughly a 1.6% chance of their baby developing early-onset GBS disease. That may sound small, but the infections can be severe, including bloodstream infections, pneumonia, and meningitis. With antibiotics, that risk drops well below 1%. This prevention strategy has dramatically reduced the number of newborn GBS infections since universal screening began.
What If You Go Into Preterm Labor
If you go into labor before your scheduled screening, your GBS status will be unknown. In that case, your provider will typically give you antibiotics during labor as a precaution, especially if other risk factors are present like prolonged rupture of membranes or fever.
If you were screened earlier because of a preterm labor scare that resolved, and then you’re readmitted with labor signs more than 5 weeks later, you’ll need a new test. The 5-week validity window applies regardless of why the original test was done.
Scheduled Cesarean Deliveries
GBS screening is still recommended at 36 to 37 weeks even if you have a planned cesarean. This is because labor can start unexpectedly before your surgery date, and your water could break ahead of schedule. If that happens, knowing your GBS status helps your team act quickly. Women who deliver by cesarean before labor begins and before their water breaks have a very low risk of transmitting GBS, but the screening ensures you’re covered if plans change.