What is a group B strep (GBS) test?
A group B strep test is a swab test that checks whether you're currently carrying group B Streptococcus, a type of bacteria that lives naturally in the digestive and genital tract of many adults. It isn't a sexually transmitted infection, and carrying it isn't a sign that anything is wrong with you or your pregnancy. Group B strep (often shortened to GBS) usually causes no symptoms at all in adults, which is exactly why testing rather than symptom-watching is how it's found. The test simply tells your provider whether the bacteria is present in your body around the time you'll give birth.
The reason this particular bacteria gets its own routine test — when most of the harmless bacteria living in your body don't — is what happens if it passes to a newborn during delivery. A baby exposed to GBS during birth can, in some cases, develop a serious infection in the first days of life. Testing late in pregnancy lets your provider know, before labor starts, whether you'd benefit from antibiotics during labor to lower that risk. It's a preventive step, not a diagnosis of any current illness in you.
When is the GBS test done during pregnancy?
The GBS test is typically done between 36 and 37 weeks of pregnancy, close enough to your due date that the result reflects what's true near delivery. GBS colonization can come and go — you might carry it at one point in pregnancy and not another — so testing earlier wouldn't reliably predict your status at birth. Testing this late means the result is usually still accurate when labor begins, whether that's on your due date or a couple of weeks either side of it.
If you go into labor before 36 weeks, before the test would normally happen, your provider will usually treat you as though your status is unknown and make a decision based on other risk factors — such as a prior baby with GBS infection, a fever during labor, or your water breaking well before delivery. If you're carrying twins or have a more complex pregnancy, your provider may adjust the timing slightly. Either way, this is a decision your midwife or doctor makes with you, not a fixed rule that applies identically to everyone.
How is the GBS test performed?
The GBS test is done with a swab, and it's quick and not painful — most people describe it as mildly uncomfortable at most, similar to a routine pelvic exam swab. Your provider (or sometimes you, with instructions) swabs the lower vagina and the rectum, since GBS can live in both areas. The swab takes only a few seconds; there's no needle, no fasting, and no special preparation beforehand.
The sample is sent to a lab, and results typically come back within a few days to about a week, depending on the lab and your clinic's turnaround. You don't need to do anything differently while waiting. If you haven't heard your result by your next appointment, it's completely reasonable to ask your provider directly rather than assuming no news is good news.
What does a positive GBS result mean?
A positive result means group B strep bacteria was found in your swab at the time of testing — it does not mean you're sick, and it does not mean your baby will be affected. Many people who test positive go on to have completely healthy deliveries and healthy babies. The result simply changes the plan for labor: instead of no action, your care team will recommend IV antibiotics once labor starts or your water breaks, given at intervals until delivery.
It's worth saying plainly because it surprises people: testing positive is common, and it is treated as a routine part of birth planning, not an emergency or a complication of your pregnancy. You don't need antibiotics before labor begins, and a positive test doesn't change anything about your prenatal appointments between now and delivery. It only becomes relevant once labor actually starts, which is when the antibiotics do their work.
What happens during labor if I test positive?
If you test positive, your provider will recommend antibiotics through an IV during labor, most often penicillin, given as soon as labor starts or your membranes rupture and then repeated every few hours until birth. This timing matters more than the total dose — starting early in labor gives the antibiotics time to reduce the amount of bacteria your baby is exposed to during delivery. If you have a known penicillin allergy, tell your provider well before labor so an alternative antibiotic can be arranged in advance rather than decided in the moment.
If you're planning a scheduled cesarean birth and haven't gone into labor or had your water break, antibiotics for GBS usually aren't needed, since the exposure they're meant to prevent happens during vaginal passage. If labor starts unexpectedly before a planned cesarean, though, the usual antibiotic protocol applies. Either way, this is something worth discussing with your provider ahead of time so you know what to expect on the day.
What if I test negative for GBS?
A negative result means the bacteria wasn't detected in your swab at 36 to 37 weeks, and for most people that means no GBS antibiotics will be needed during labor. Because colonization can change, a small number of people who test negative are found to be positive by the time labor starts — this is uncommon, but it's part of why some risk factors during labor (like a fever) are still taken seriously even after a negative test.
If you have a fever during labor, if your water breaks and stays broken for an extended period before delivery, or if you previously had a baby affected by GBS infection, your provider may still recommend antibiotics regardless of your test result. These are precautions based on the overall picture, not a sign that your test was wrong or that something has gone unnoticed. Your care team will explain their reasoning if this applies to you.
Can GBS harm my baby, and what symptoms should I watch for after birth?
Untreated GBS exposure during birth can, in a minority of cases, lead to a newborn infection affecting the blood, lungs, or, less often, the brain and spinal cord — which is exactly what labor antibiotics are designed to prevent. Most babies exposed to GBS during delivery, especially when antibiotics were given during labor, do not develop an infection. Your hospital team will typically watch your baby closely in the first day or two after birth if you tested positive, even after antibiotics.
Call your baby's provider right away, or go to the emergency department, if your newborn has a fever, unusual fussiness or lethargy, trouble breathing, feeding poorly, or a noticeably blue or pale tone to the skin — these can appear in the first week of life and warrant same-day evaluation. Early GBS infection usually shows up within the first 24 to 48 hours; late-onset infection can appear up to a few months after birth, though it's less common. Trust your instincts here — you know your baby, and a same-day call is always the right move if something feels wrong.