A GBS swab is a simple screening test done late in pregnancy to check whether you carry Group B Streptococcus, a common bacterium that’s usually harmless in adults but can cause serious infections in newborns during delivery. The test is recommended for all pregnant people during the 36th or 37th week of each pregnancy, and it involves a quick swab of the vaginal and rectal area.
What Group B Strep Actually Is
Group B Streptococcus (GBS) is a type of bacteria that naturally lives in the digestive and reproductive tracts of many adults without causing any symptoms. It comes and goes on its own, and carrying it doesn’t mean you have an infection or that anything is wrong. Roughly 15 to 20 percent of pregnant people are colonized with GBS at any given time.
The concern isn’t about you. It’s about your baby. During a vaginal delivery, GBS bacteria can pass to the newborn. In rare cases, this leads to serious illnesses like bloodstream infections, pneumonia, or meningitis. About 20 to 30 percent of babies who survive GBS meningitis may have lasting problems such as hearing loss, seizures, or delays in learning to move, speak, or think. That’s why the screening exists: to identify carriers ahead of delivery so preventive treatment can be given.
How the Swab Is Collected
The test itself takes only a few seconds. A single swab is used to collect a sample from two areas: first the lower vagina (just inside the vaginal opening), then the rectum, inserting the swab just past the anal sphincter. Both sites are swabbed because GBS bacteria can live in either location, and testing both gives the most accurate result.
Your provider can do the swab, or you can do it yourself. Research has found no difference in accuracy between self-collected and provider-collected swabs. If your provider offers self-collection, you’ll typically receive written instructions and a diagram showing exactly where to swab. Many people find self-collection more comfortable, especially for the rectal portion.
The swab feels similar to a basic vaginal exam. There’s no speculum involved, and most people describe it as mildly uncomfortable rather than painful.
Why It’s Done So Late in Pregnancy
Because GBS bacteria come and go naturally in your body, you might test positive one month and negative the next. Screening is timed between 36 weeks 0 days and 37 weeks 6 days of gestation specifically to capture your colonization status as close to delivery as possible. This timing gives a five-week window during which your results remain valid, covering births that happen up to at least 41 weeks.
If you were tested earlier in pregnancy, the result wouldn’t reliably predict whether you’d still be carrying the bacteria at the time of delivery. You also get screened with every pregnancy, not just your first, since your GBS status can change between pregnancies.
Getting Your Results
The swab is sent to a lab where the sample is cultured to see if GBS bacteria grow. Most people receive their results within 48 hours. The result is straightforward: positive or negative.
Once you test positive, you’re considered GBS-positive for the rest of that pregnancy. A positive result doesn’t mean anything is wrong with you or that you need treatment right away. It simply flags that you’ll need antibiotics during labor to prevent transmission to your baby.
What Happens if You Test Positive
If your swab comes back positive, your care team will plan to give you intravenous antibiotics once labor begins. The antibiotics work by reducing the amount of GBS bacteria in your birth canal during delivery, which dramatically lowers the chance of your baby being exposed.
The antibiotics are given through an IV, typically starting when you arrive at the hospital in active labor or when your water breaks. Ideally, you receive the first dose at least four hours before delivery to give the medication enough time to work. This is why it’s important to let your provider know promptly when labor starts.
If you have a penicillin allergy, mention it when your GBS culture is ordered. The lab can then test the bacteria for sensitivity to alternative antibiotics, so your care team has the right backup plan ready.
GBS Infection in Newborns
GBS disease in infants takes two forms. Early-onset disease appears within the first week after birth and typically causes bloodstream infections or pneumonia. Late-onset disease develops between 7 and 89 days after birth and more commonly causes meningitis. The screening and antibiotic strategy during labor is designed to prevent early-onset disease specifically.
These infections are serious but uncommon, and universal screening has made them rarer still. The combination of identifying carriers and treating them during labor is one of the most effective preventive measures in newborn medicine.
If You Go Into Labor Before Being Tested
If you deliver before your scheduled screening, either because labor starts early or because the test wasn’t done, your care team will assess your risk factors during labor and may give you antibiotics as a precaution. Situations that increase the likelihood of GBS transmission include preterm labor, prolonged time between your water breaking and delivery, and fever during labor. Your provider will make the call based on these factors.