A Group B Strep (GBS) culture is a lab test that checks whether a specific type of bacteria, Group B Streptococcus, is living in your body. It’s most commonly performed during pregnancy, between 35 and 37 weeks, using swabs from the vagina and rectum. The test determines whether you carry GBS so your medical team can take steps to protect your baby during delivery.
GBS is a normal bacterium that resides in the gastrointestinal and genital tracts of up to one-third of healthy women without causing any symptoms. Carrying it doesn’t mean you’re sick or have an infection. But because GBS can pass to a baby during birth and cause serious illness, routine screening has become a standard part of prenatal care.
Why GBS Screening Matters in Pregnancy
In newborns, GBS is a leading cause of sepsis (a bloodstream infection), pneumonia, and meningitis. These are life-threatening conditions that can result in death or long-term disabilities. Among infants born to mothers who carry GBS and don’t receive treatment during labor, 40 to 75% of those babies will pick up the bacteria, and roughly 12% of colonized infants will develop early-onset invasive GBS disease.
Those numbers drop dramatically when the mother receives antibiotics through an IV during labor. Universal screening at 35 to 37 weeks, combined with antibiotics given during delivery, has substantially reduced the rate of early-onset GBS disease in newborns. That’s why the test is offered to virtually all pregnant women, regardless of how healthy they feel.
How the Swab Is Collected
The test itself is quick and straightforward. Your provider uses two swabs: one collects a sample from the lower vagina (the vaginal opening, not higher up with a speculum), and the second swabs the rectum. The rectal swab is important because the gastrointestinal tract is GBS’s primary home in the body, so skipping it would miss many carriers. Some practices use a single swab for both sites. The whole process takes just a few seconds and feels similar to a routine swab, with mild pressure but rarely any real pain.
Many providers will let you collect the swabs yourself if you prefer, since self-collection has been shown to be just as accurate.
What Happens in the Lab
The swabs are placed in a nutrient broth and incubated at body temperature. Lab technicians check for bacterial growth at 24 hours. If no growth is visible, the sample incubates for another 24 hours before being called negative. So results typically take one to two days, though your provider may not contact you for a few days after that.
A positive result means GBS was growing in the culture. It doesn’t mean you have an infection. It simply means you’re colonized, carrying the bacteria at the time the sample was taken. GBS colonization can come and go, which is why the test is done late in pregnancy: the goal is to capture your status as close to delivery as possible.
Culture vs. Rapid Testing
The traditional culture has long been considered the gold standard, but newer molecular tests (called nucleic acid amplification tests, or NAATs) are gaining ground. These tests detect GBS DNA rather than waiting for bacteria to grow, so they return results much faster, sometimes within an hour.
In a study comparing three rapid molecular tests against the standard culture using 500 vaginal-rectal swab specimens, all 108 culture-positive samples were also caught by at least one rapid test. The rapid tests actually flagged an additional 47 specimens that culture missed, suggesting they may be more sensitive. Rapid tests are particularly useful when a woman arrives in labor without prior screening results, since there’s no time to wait for a culture to grow.
What a Positive Result Means for Delivery
If your GBS culture comes back positive, the standard approach is to receive antibiotics through an IV once labor begins. This is called intrapartum antibiotic prophylaxis. The antibiotics work by reducing the amount of GBS bacteria in the birth canal so the baby is far less likely to be exposed during delivery. They’re most effective when given at least four hours before birth, which is why your provider will want to start them early in active labor.
About 18% of pregnant women worldwide carry GBS, so a positive result is common and not a cause for alarm. You won’t need to take oral antibiotics during pregnancy. Treatment before labor isn’t effective because GBS recolonizes quickly from the gut. The only window that matters is the hours surrounding delivery itself.
A positive result also doesn’t change how or where you can deliver. It simply adds the IV antibiotic step to your birth plan. If you have a scheduled cesarean section before labor begins and before your water breaks, antibiotics for GBS are generally not needed, since the baby won’t pass through the birth canal.
GBS Testing Outside of Pregnancy
While pregnancy screening is by far the most common reason for a GBS culture, the test is sometimes used in other situations. GBS can cause infections in adults, particularly older adults, people with diabetes, and those with weakened immune systems. In these cases, GBS cultures may be ordered from blood, urine, wound sites, or spinal fluid depending on where the infection is suspected.
Symptoms of GBS infection in adults vary by location. A bloodstream infection causes fever, chills, and low alertness. A lung infection brings cough, difficulty breathing, and chest pain. Skin and soft tissue infections appear as red, swollen, warm areas that may drain pus. Bone and joint infections cause localized pain, swelling, and stiffness. These infections are treated with antibiotics tailored to the site and severity, and they are distinct from the harmless colonization that pregnancy screening detects.