Group B strep (GBS) is a type of bacteria called Streptococcus agalactiae that naturally lives in the digestive and genital tracts of many healthy adults. Roughly 1 in 4 pregnant women carry it at any given time without symptoms or health problems. It becomes medically important primarily during pregnancy and childbirth, when it can pass to a newborn and cause serious infection.
Where GBS Lives in the Body
GBS is an opportunistic organism, meaning it coexists harmlessly with the body’s other bacteria most of the time. It commonly colonizes the intestines, vagina, and rectum. Carrying GBS is not an infection, and it doesn’t mean anything is wrong. Colonization can come and go on its own, appearing during one period of life and disappearing during another. This is why screening during pregnancy is timed close to the due date rather than done early on.
Why It Matters During Pregnancy
The primary concern with GBS is the risk of passing the bacteria to a baby during labor and delivery. A newborn’s immune system is not yet equipped to fight off certain bacterial infections, and GBS can cause life-threatening illness in the first days or months of life. The bacteria can be transmitted as the baby moves through the birth canal or, less commonly, through ruptured membranes before delivery.
To reduce this risk, all pregnant women are screened for GBS between 36 and 37 weeks of gestation, regardless of how they plan to deliver. The test is simple: a provider uses a small cotton swab to collect a sample from the vagina and rectum. Results typically come back within a day or two. A positive result doesn’t mean you’re sick. It just means the bacteria are present and that preventive steps should be taken during labor.
If you test positive, you’ll receive antibiotics through an IV during labor. This is the most effective way to prevent the bacteria from reaching the baby during delivery. The antibiotics work best when given at least four hours before birth, which is one reason providers encourage coming to the hospital early once labor begins. If you have a penicillin allergy, alternative antibiotics are available, and your provider will choose one based on sensitivity testing of the bacteria from your swab.
There are a few situations where screening isn’t necessary because antibiotics are already indicated: if GBS was found in your urine at any point during the pregnancy, or if you previously had a baby who developed a GBS infection.
GBS Infection in Newborns
GBS disease in newborns falls into two categories based on timing. Early-onset disease occurs in the first 6 days of life, and most babies who develop it show symptoms on the day they’re born. Late-onset disease appears between 7 and 89 days after birth. Early-onset disease is the type that prenatal screening and labor antibiotics are designed to prevent. Late-onset disease is less well understood and not reliably prevented by the same measures.
Symptoms in a newborn with GBS infection include fever, difficulty breathing, a bluish tint to the skin, difficulty feeding, and unusual limpness or difficulty waking the baby. These signs can escalate quickly. The most common forms of GBS disease in newborns are bloodstream infections (sepsis), pneumonia, and meningitis.
Thanks to routine screening and preventive antibiotics during labor, early-onset GBS disease has dropped dramatically since the 1990s. But it hasn’t been eliminated entirely, which is why the screening protocol remains a standard part of prenatal care.
GBS Infections in Adults
While the conversation around GBS focuses heavily on pregnancy and newborns, the bacteria can also cause illness in non-pregnant adults, particularly older adults and people with chronic health conditions like diabetes, heart disease, or liver disease. In these groups, GBS most commonly causes bloodstream infections, pneumonia, and skin or bone infections. Meningitis from GBS is very uncommon in adults.
Adults who develop GBS infections are typically already dealing with another condition that has weakened their immune defenses. For otherwise healthy adults, carrying GBS is not a cause for concern and doesn’t require treatment.
What a Positive GBS Result Means for You
If you’ve tested positive for GBS during pregnancy, the most important thing to understand is that this is a routine finding, not a diagnosis of disease. You won’t feel any different, and GBS colonization doesn’t cause symptoms like discharge, pain, or odor. It’s simply a signal that a precaution is needed during delivery.
Because GBS colonization can fluctuate, a positive result from an earlier pregnancy doesn’t automatically mean you’ll test positive in a subsequent one. Each pregnancy includes a fresh screening at 36 to 37 weeks. If you go into preterm labor before you’ve been screened and your GBS status is unknown, your provider will typically give antibiotics as a precaution.
Outside of pregnancy, most people never learn their GBS status and don’t need to. The bacteria are part of normal human flora for a significant portion of the population, and for the vast majority, they cause no harm at all.