Group B strep screening happens between 36 weeks 0 days and 37 weeks 6 days of pregnancy. This is a universal screening, meaning every pregnant person gets it regardless of whether they plan a vaginal birth or a cesarean delivery. The test is quick, and the timing is deliberately chosen so results stay valid through your due date and a bit beyond.
Why the 36 to 37 Week Window
The screening window used to be 35 to 37 weeks, but the American College of Obstetricians and Gynecologists (ACOG) updated the recommendation to start at 36 weeks instead. The reason comes down to how long results remain reliable. A GBS culture is considered valid for five weeks after collection. By swabbing at 36 to 37 weeks, your results cover births up to at least 41 weeks of gestation, which accounts for the vast majority of deliveries, including those that go past the due date.
This change also means fewer people need repeat testing. Under the old window, someone swabbed at 35 weeks whose pregnancy stretched to 41 weeks might have needed a second culture. The updated timing largely eliminates that problem.
What the Swab Involves
The test is a simple swab of the lower vagina and rectum. A healthcare provider uses a single swab, first collecting from the vaginal opening and then from the rectum. It takes just a few seconds and feels similar to a standard vaginal swab, though the rectal portion can be mildly uncomfortable. No speculum is needed.
In many settings, you can collect the sample yourself. Research comparing self-collected swabs to those taken by a clinician has found that self-collection works well for detecting GBS, and some providers now offer this as an option. If you’d prefer to do the swab yourself, it’s worth asking your provider whether they support that at your practice.
The swab is sent to a lab for culture, and results typically come back within 24 to 48 hours.
What Group B Strep Actually Is
Group B Streptococcus (GBS) is a common bacterium that roughly 10 to 40 percent of pregnant women carry in their vaginal or rectal area. It’s not a sexually transmitted infection and doesn’t mean anything is wrong with you. Most adults who carry it have no symptoms at all. The bacteria can come and go on its own, which is exactly why the test is done late in pregnancy rather than earlier: a result from 20 weeks wouldn’t reliably predict your status at delivery.
The concern isn’t about the pregnant person. It’s about the baby picking up the bacteria during a vaginal birth. In newborns, GBS is a leading cause of serious infections including sepsis (a bloodstream infection), pneumonia, and meningitis. These infections can develop within the first week of life or, less commonly, up to about three months after birth. About 3 in 10 babies who recover from GBS meningitis may have lasting effects like hearing loss, seizures, or developmental delays.
What Happens if You Test Positive
A positive result means you’ll receive IV antibiotics during labor. The antibiotics are given through your IV line once labor begins or your water breaks, and they work best when administered at least four hours before delivery. This approach reduces the risk of early-onset GBS infection in the newborn by about 80 percent.
Testing positive doesn’t change anything about your pregnancy before labor starts. You won’t take oral antibiotics ahead of time, because clearing the bacteria before delivery doesn’t prevent recolonization by the time you give birth. The strategy is specifically about having antibiotics on board during the hours when the baby is most exposed.
There is one notable exception: if you’re having a planned cesarean delivery and your water hasn’t broken before surgery, GBS antibiotics during the procedure aren’t typically needed. The baby doesn’t pass through the birth canal in that scenario, so the main route of exposure isn’t present. That said, you still get screened at 36 to 37 weeks regardless of your birth plan, because plans can change. If labor starts unexpectedly or your water breaks before a scheduled cesarean, knowing your GBS status lets your care team act quickly.
When You Skip Screening Entirely
There are two situations where the standard swab at 36 to 37 weeks is unnecessary. If GBS was found in your urine at any point during the current pregnancy (even if it was treated), you’re automatically considered positive and will receive antibiotics in labor without a separate swab. The same applies if you previously had a baby who developed a GBS infection. In both cases, your provider already knows you need treatment and screening wouldn’t change the plan.
If Labor Comes Early
Going into labor before 36 weeks means you likely haven’t been screened yet. In that case, a GBS swab is collected at the time of hospital admission. If your colonization status is unknown and you’re showing signs of true preterm labor, antibiotics are started right away rather than waiting for culture results. If the culture comes back negative, the antibiotics are stopped. If it turns out you’re not in actual labor, they’re also discontinued.
For anyone whose water breaks early but labor hasn’t started, the approach depends on whether you’ve had a GBS screen within the past five weeks. If you haven’t, or if it was positive, you’ll generally receive a short course of antibiotics while your care team monitors the situation. The goal is the same as with full-term deliveries: reduce the baby’s exposure to the bacteria during birth.