A group B strep test is a simple swab done late in pregnancy to check whether you carry a common bacterium that could pass to your baby during delivery. The test is recommended for all pregnant women between 36 and 37 weeks of gestation, and the results determine whether you’ll receive antibiotics during labor to protect your newborn.
What Group B Strep Is
Group B streptococcus (GBS) is a bacterium that naturally lives in the vaginal and rectal areas of many adults without causing any symptoms. Roughly 15 to 40 percent of pregnant women carry GBS at any given time, depending on geographic region. Carrying GBS doesn’t mean you have an infection or that anything is wrong with your health. Most of the time, the bacterium is completely harmless to adults.
The concern is specific to childbirth. During a vaginal delivery, GBS can transfer from the birth canal to the baby. A newborn’s immune system is not equipped to fight off these bacteria the way an adult’s can, and in a small percentage of cases, GBS causes serious illness in infants: bloodstream infections (sepsis), lung infections (pneumonia), or infection of the membranes surrounding the brain (meningitis). Globally, GBS affects an estimated 390,000 infants each year and is the leading cause of acute bacterial meningitis in newborns, according to the World Health Organization.
How the Test Works
The test itself takes about 10 seconds. Your provider uses a single swab to collect a sample from the lower vagina (about 2 centimeters inside the opening) and then from the rectum (about 1 centimeter inside). No speculum is needed, and most women describe it as mildly uncomfortable at most. In many practices, you can even do the swab yourself with instructions from your provider.
The swab is sent to a lab where it’s placed in a culture medium to see if GBS bacteria grow. Results typically come back within 24 to 48 hours. A positive result means the bacterium was detected; a negative result means it wasn’t found at the time of testing. GBS colonization can come and go, which is why the timing of the test matters.
When the Test Is Done
Current guidelines from the American College of Obstetricians and Gynecologists recommend screening between 36 weeks 0 days and 37 weeks 6 days of pregnancy. This window was chosen carefully. Culture results remain valid for about five weeks, so testing during this period covers deliveries that happen up through at least 41 weeks. Testing too early in pregnancy is unreliable because your GBS status can change between then and delivery.
If you go into preterm labor before you’ve been screened, or if your results aren’t available when labor starts, your care team will make decisions based on other risk factors like whether you have a fever or how long your water has been broken.
What a Positive Result Means
Testing positive for GBS does not mean you’re sick or that your baby will definitely be affected. It simply means the bacterium is present and there’s a chance it could transfer during delivery. The standard approach is to give you IV antibiotics once labor begins. The preferred medication is penicillin, with a closely related antibiotic as an alternative. If you have a penicillin allergy, your provider will choose a different antibiotic based on the severity of your allergy and, in some cases, lab testing to see which drugs will work against your specific strain of GBS.
Timing matters. The antibiotics work best when given at least four hours before delivery. That four-hour mark is considered the threshold for “adequate” treatment, meaning the medication has had enough time to reach effective levels in the birth canal and reduce the bacteria your baby encounters. This is why providers start antibiotics as soon as active labor is confirmed or your water breaks, rather than waiting until delivery is imminent. You’ll continue to receive doses every few hours throughout labor until your baby is born.
The antibiotics are given through an IV, so you’ll have a line placed when you arrive at the hospital or birthing center. Between doses, the line can be capped so you’re free to move around.
Why Screening Has Made a Difference
Before universal screening became standard practice, early-onset GBS disease in newborns was significantly more common. A large population-based study in Ontario found that maternal screening was associated with a 40 percent reduction in infant GBS disease overall. Early-onset infections, those occurring in the first week of life, have declined substantially since screening programs were introduced. Late-onset disease, which can appear from 7 to 89 days after birth, has remained more stable because it’s not as directly linked to the birth canal exposure that antibiotics target.
What Happens to the Baby After Delivery
If you tested positive and received antibiotics for at least four hours before delivery, your baby will typically be monitored with routine newborn care. No additional testing or treatment is usually needed for the infant.
If antibiotics were given for less than four hours, or if your GBS status was unknown at the time of delivery, your baby may be observed more closely for signs of infection during the first 24 to 48 hours. These signs include difficulty breathing, unusual fussiness or lethargy, feeding problems, and unstable temperature. In most cases, babies do perfectly well, but the extra monitoring provides an early warning if intervention is needed.
Scheduled Cesarean Deliveries
If you’re having a planned cesarean section before labor starts and before your water breaks, your baby won’t pass through the birth canal. In this scenario, GBS-specific antibiotics during the procedure are generally not needed regardless of your screening result. You’ll still receive the standard antibiotics that are given before any cesarean to prevent surgical-site infections, but those serve a different purpose. If labor begins unexpectedly before your scheduled cesarean, or if your water breaks beforehand, the usual GBS protocol applies.
GBS Found in Urine During Pregnancy
Sometimes GBS shows up in a routine urine culture earlier in pregnancy, even if you have no urinary symptoms. If GBS is detected in your urine at any point during the pregnancy, you’re automatically considered GBS-positive for delivery purposes. You won’t need the late-pregnancy swab because the urine finding already indicates a higher level of colonization. Your provider will note this in your chart so that antibiotics are given when labor begins.
What You Can’t Control
There’s no way to permanently clear GBS from your body before delivery. The bacterium is part of normal gut and vaginal flora and can fluctuate on its own. Dietary changes, probiotics, and herbal remedies have not been shown to reliably eliminate GBS colonization in time for delivery. The screening-plus-antibiotics approach remains the most effective strategy for protecting newborns, and it works well for the vast majority of GBS-positive pregnancies.