Group B Streptococcus (GBS) is very common. Up to 35% of healthy women carry the bacteria in their gastrointestinal or genital tracts at any given time, usually without any symptoms at all. For most people, GBS is a harmless part of the body’s normal bacterial environment. It only becomes a medical concern in specific situations, primarily during pregnancy and childbirth, or in adults with weakened immune systems.
Carrying GBS vs. Having an Infection
The distinction between colonization and infection is key to understanding GBS. Colonization means the bacteria live in your body, typically in the vagina, rectum, or lower intestines, without causing any harm. Up to one in three healthy women are colonized at any point. This isn’t a disease, and it doesn’t need treatment on its own. Colonization can also come and go over time, so someone who tests positive during one period may test negative later.
An active GBS infection is far less common. It occurs when the bacteria move beyond their usual location and invade the bloodstream, lungs, or other tissues. This is most dangerous for newborns, whose immune systems aren’t yet equipped to fight off the bacteria. In adults outside of pregnancy, invasive GBS infections are relatively rare and tend to occur in people with other health conditions like diabetes, liver disease, or compromised immunity.
Why GBS Matters During Pregnancy
GBS becomes a specific concern during labor and delivery because the bacteria can pass from the birth canal to the baby. Before preventive measures were widely adopted, early-onset GBS disease in newborns occurred at a rate of about 1.8 cases per 1,000 live births in the early 1990s. That number has dropped dramatically thanks to routine screening and treatment, falling to roughly 0.26 cases per 1,000 live births by 2010, a reduction of more than 80%.
The American College of Obstetricians and Gynecologists recommends that all pregnant women be screened for GBS between 36 and 37 weeks of gestation, regardless of how they plan to deliver. The test involves a simple swab of the vagina and rectum, which is sent for a bacterial culture. This culture method has about 81% sensitivity and near-perfect specificity, meaning it occasionally misses a positive case but very rarely gives a false positive.
If you test positive, you’ll receive antibiotics through an IV during labor. When given at least four hours before delivery, this approach is about 91% effective at preventing early-onset GBS disease in full-term infants and 86% effective in preterm infants. Women who had GBS in their urine during pregnancy or who previously had a baby affected by GBS automatically receive antibiotics during labor without needing the late-pregnancy swab.
How GBS Affects Newborns
GBS disease in infants comes in two forms. Early-onset disease appears within the first week of life, often within 24 to 48 hours of birth. It can cause bloodstream infections, pneumonia, and meningitis. Current data from a large population-based study in Ontario, Canada, puts the early-onset rate at about 0.49 per 1,000 live births in a screened population.
Late-onset disease develops between 7 and 89 days after birth. It tends to progress more slowly and is particularly associated with meningitis. The late-onset rate is similar, at roughly 0.46 per 1,000 live births. Unlike early-onset disease, late-onset GBS isn’t always transmitted during delivery. Babies can pick up the bacteria from caregivers or from hospital environments after birth, which is why good hygiene practices (handwashing, clean surfaces) play an important role in prevention.
GBS in Adults Outside of Pregnancy
GBS isn’t exclusively a pregnancy concern. It can cause infections in any adult, though this is uncommon in otherwise healthy people. The most frequent GBS infections in non-pregnant adults are bloodstream infections, pneumonia, and skin and bone infections. Urinary tract infections from GBS also occur. Meningitis caused by GBS is very rare in adults.
Risk increases with age and with chronic conditions that suppress the immune system. Adults over 65, people with diabetes, and those with liver or heart disease face the highest risk. For most healthy adults, though, carrying GBS is no different from carrying any other common bacterium. It doesn’t require treatment and won’t cause problems.
What a Positive GBS Test Means for You
If you’ve tested positive for GBS during pregnancy, it helps to know that you’re in a large group. Roughly one in three women will test positive at some point. A positive result doesn’t mean anything went wrong, and it doesn’t indicate poor hygiene or an infection you “caught” from someone. It simply means the bacteria are present in your body right now, and your care team will take a straightforward step during labor to protect your baby.
GBS colonization status can change between pregnancies and even within the same pregnancy, which is why screening happens late in the third trimester rather than earlier. A result from 30 weeks, for example, might not accurately reflect your status at delivery. If you go into labor before being screened, or if your results aren’t available, your medical team will consider risk factors like preterm labor, prolonged rupture of membranes, or fever during labor to decide whether to give preventive antibiotics.