Group B streptococcus (GBS) is a type of bacteria that naturally lives in the digestive and genital tracts of roughly 1 in 4 healthy adults. Most people who carry it never get sick from it. The bacterium becomes medically significant in two situations: during pregnancy, when it can pass to a newborn during birth, and in older adults or people with chronic health conditions, where it can cause serious infections.
Where GBS Lives in the Body
GBS, known scientifically as Streptococcus agalactiae, is what microbiologists call an opportunistic commensal. That means it lives quietly as part of your normal gut bacteria without causing harm. The gastrointestinal tract is its primary home and likely the source from which it spreads to the vaginal and urinary tracts in women. Colonization is typically intermittent: it can come and go over months or years without any symptoms.
Carrying GBS doesn’t mean you have an infection. It means the bacteria is present on your body’s mucosal surfaces, much like dozens of other harmless organisms. The distinction matters because colonization only becomes a concern under specific circumstances.
How GBS Affects Newborns
The primary reason most people hear about GBS is pregnancy. A colonized mother can pass the bacteria to her baby in two ways: the organism can travel upward from the vagina into the amniotic fluid before or during labor, or the baby can pick it up while moving through the birth canal. Most infants exposed this way simply become colonized on their skin or mucous membranes and stay perfectly healthy. In a small percentage of cases, however, the bacteria enters the baby’s bloodstream and causes serious illness.
Newborn GBS disease comes in two forms. Early-onset disease strikes within the first week of life and most often causes blood infections (sepsis) or pneumonia. Late-onset disease appears between 7 and 89 days after birth and is more likely to involve meningitis, an infection of the membranes surrounding the brain. Warning signs in either case include fever, poor feeding, lethargy, irritability, difficulty breathing, and bluish or pale skin. Meningitis may also cause seizures or a noticeably bulging soft spot on the baby’s head.
Screening During Pregnancy
Because GBS colonization causes no symptoms in the mother, the only way to know your status is through a screening test. The American College of Obstetricians and Gynecologists and the American College of Nurse-Midwives recommend universal screening during the 36th or 37th week of each pregnancy. The test is simple: a healthcare provider uses a sterile swab on the vagina and rectum, then sends the sample to a lab. Results typically come back within a day or two.
Testing happens late in pregnancy because colonization fluctuates. A negative result at 20 weeks doesn’t guarantee you’ll still be negative at delivery. The 36- to 37-week window gives the most accurate picture of your status when labor begins.
Preventing Transmission During Labor
If you test positive for GBS, the standard prevention strategy is intravenous antibiotics given during labor. The goal is to reduce the amount of bacteria present in the birth canal so the baby encounters far less of it. Timing is critical: antibiotics given at least 4 hours before delivery are highly effective at preventing transmission. When antibiotics are started closer to the time of birth, the protective effect drops significantly.
After delivery, a baby whose mother received adequate antibiotic prophylaxis (4 or more hours before birth) is typically observed for at least 48 hours without needing any special testing. If antibiotics were given for a shorter window or not at all, the medical team will monitor the baby more closely and may run additional tests.
For women with penicillin allergies, alternative antibiotics are available, and your provider will determine the best option based on the severity of your allergy. Letting your care team know about any drug allergies well before your due date helps them plan accordingly.
GBS Infections in Adults
Newborns get most of the attention, but GBS also causes invasive infections in non-pregnant adults, particularly older people and those with underlying health conditions. The most common presentations are bloodstream infections without an obvious source, skin and soft tissue infections, urinary tract infections, and pneumonia.
The adults most vulnerable tend to have one or more chronic conditions. Diabetes is especially common among those who develop GBS skin infections. Heart disease, kidney disease, liver disease, neurological disorders, obesity, and conditions that suppress the immune system all increase risk. Alcohol abuse and smoking are additional contributing factors. For a healthy younger adult, the odds of developing an invasive GBS infection are very low.
GBS Vaccine Development
There is currently no approved vaccine against GBS, but several candidates designed for use during pregnancy are moving through clinical trials. The idea is similar to other maternal vaccines: immunize the mother so that protective antibodies cross the placenta and shield the newborn during its most vulnerable weeks. Researchers and regulators, including the World Health Organization, are actively discussing whether these vaccines could be approved based on their ability to generate a strong immune response rather than requiring enormous trials measuring actual disease prevention, which is difficult given how relatively rare neonatal GBS disease is in any single study population. If successful, a maternal GBS vaccine could eventually reduce or replace the need for antibiotic use during labor.