About 1 in 4 pregnant women carry group B strep (GBS) at any given time, making it one of the most common bacteria found in the human body. Despite how widespread it is, GBS rarely causes serious problems in healthy adults. The real concern centers on newborns, older adults, and people with chronic health conditions, where the bacteria can turn from a harmless colonizer into a dangerous infection.
Colonization Rates in Pregnant Women
GBS lives naturally in the vagina and rectum, and roughly 25% of pregnant women are colonized at the time of screening. “Colonized” means the bacteria are present but not causing an infection or symptoms. You can carry GBS during one pregnancy and not the next, because colonization fluctuates over weeks and months. It isn’t a sexually transmitted infection, and having it doesn’t reflect anything about hygiene or health habits.
Because colonization is so common and so variable, screening happens late in pregnancy, typically between 36 and 37 weeks. A simple swab of the vagina and rectum gives results within a day or two. The timing matters: a test done earlier in pregnancy may not reflect your status at delivery.
How Often Babies Actually Get Sick
Carrying GBS is common, but newborn infection is not. Among babies born to colonized mothers who don’t receive antibiotics during labor, about 50% come into contact with the bacteria during delivery. Of those exposed babies, only 1 to 2 out of 100 go on to develop a serious infection in their first days of life. That’s a small fraction of an already small fraction.
The current rate of early-onset GBS disease in the United States is about 0.23 per 1,000 live births. That number has dropped dramatically, down more than 80% from the 1990s, when the rate was 1.8 per 1,000 live births. The decline is almost entirely due to universal screening and the practice of giving IV antibiotics during labor to mothers who test positive.
Late-onset GBS disease, which appears between one week and about three months after birth, occurs at a rate of roughly 0.37 per 1,000 live births. Unlike early-onset disease, late-onset infections aren’t clearly prevented by antibiotics given during labor. The bacteria may reach the baby through breast milk, skin-to-skin contact, or other household exposure. Breast milk can carry the same GBS strain found in the baby, though researchers still debate whether the milk is the true source of infection or simply reflects shared bacteria within the family.
GBS in Non-Pregnant Adults
GBS isn’t just a pregnancy concern. Invasive GBS disease affects non-pregnant adults at a rate of about 2.86 per 100,000 people per year. That number climbs steeply with age: to roughly 9 per 100,000 in adults over 50, and about 19 per 100,000 in adults over 65. Half of all fatal GBS infections occur in the elderly.
Certain chronic conditions dramatically increase risk. People with diabetes face up to a tenfold higher risk of invasive GBS infection, and cancer raises the risk even further, up to about 16 times that of the general population. Obesity is another significant factor. One analysis estimated that diabetes accounted for about 40% of invasive GBS cases in adults, while obesity accounted for about 27%. Adults with chronic medical conditions overall have GBS infection rates two to six times higher than the general population.
The overall death rate for invasive GBS in non-pregnant adults is close to 10%, making it a serious infection when it does occur. It can cause bloodstream infections, pneumonia, skin and soft tissue infections, and bone or joint infections.
How Screening and Prevention Work
If you test positive for GBS during pregnancy, the standard approach is IV antibiotics during labor, most commonly penicillin. This doesn’t eliminate the bacteria from your body permanently. It reduces the bacterial load in the birth canal at the moment your baby passes through, which is when transmission is most likely.
For women with a serious penicillin allergy (the kind that risks anaphylaxis), clindamycin is the typical alternative. However, resistance to this antibiotic has been climbing. Nationally, about 52.5% of GBS strains show resistance to clindamycin, and some regions report rates above 70%. This is why labs routinely test GBS samples for antibiotic sensitivity when the patient has a penicillin allergy, so your care team knows which alternatives will actually work.
There is no approved GBS vaccine, though several are in development. For now, the screening-and-antibiotics strategy remains the primary tool, and its track record is strong: that 80% reduction in early-onset disease represents thousands of prevented infections each year in the U.S. alone.
What Colonization Means for You
A positive GBS test during pregnancy doesn’t mean you’re sick, and it doesn’t mean your baby will get sick. It means you’re in the same category as about one in four pregnant women, and that your delivery plan will include IV antibiotics as a precaution. Most GBS-positive mothers who receive antibiotics during labor have completely healthy babies.
Outside of pregnancy, GBS colonization is similarly common and similarly harmless for most people. It becomes a concern primarily when the immune system is compromised by age, diabetes, cancer, or other chronic illness. If you fall into one of those higher-risk groups, unusual symptoms like sudden fever, confusion, or rapidly worsening skin infections warrant prompt medical attention, since early treatment significantly improves outcomes.