Group B Streptococcus (GBS) is one of the most common bacterial findings in pregnancy. Roughly 10 to 40 percent of pregnant women carry the bacteria in their vaginal or rectal area at any given time, making it a routine part of prenatal screening rather than a rare diagnosis. Carrying GBS does not mean you or your baby will get sick, but understanding the numbers helps put the screening process and any positive result in perspective.
How Many Pregnant Women Carry GBS
GBS colonization rates vary by region. In Western Europe, about 11 to 21 percent of pregnant women test positive. Scandinavian countries see higher rates, between 24 and 36 percent. Eastern Europe falls in the 19 to 29 percent range, while Southern Europe has the widest spread at 6.5 to 32 percent. In the United States, estimates generally land around 25 percent, meaning about 1 in 4 pregnant women will test positive during screening.
Colonization is not an infection. The bacteria live naturally in the digestive and genital tracts and can come and go over weeks or months. You can test positive during one pregnancy and negative in the next, which is why screening happens with every pregnancy rather than relying on a previous result.
What Happens if GBS Reaches the Baby
About half of women who carry GBS will pass the bacteria to their newborn during vaginal delivery. That sounds alarming, but most of those babies simply become temporarily colonized without any symptoms. Without preventive treatment, 1 to 2 percent of colonized newborns develop actual GBS disease. Put another way, if you test positive and receive no treatment during labor, your baby has roughly a 1 in 200 chance of becoming ill.
GBS disease in newborns takes two forms. Early-onset disease appears within the first six days of life, most often within hours of birth, and can cause bloodstream infections, pneumonia, or meningitis. Late-onset disease develops between 7 and 89 days after birth and is less directly tied to delivery. As of 2023, the overall infection rate in the U.S. is about 0.2 cases per 1,000 live births, a number that has dropped dramatically since routine screening and preventive antibiotics became standard practice in the late 1990s.
When and How You Get Tested
The standard recommendation is to get screened during your 36th or 37th week of pregnancy. The test is a simple swab of the vagina and rectum, done in a few seconds during a regular prenatal visit. Results typically come back within a day or two. Because GBS colonization can fluctuate, testing earlier in pregnancy is not reliable for predicting your status at delivery.
If you had a previous baby affected by GBS disease, or if a urine culture during your current pregnancy detected GBS, you’ll be treated during labor automatically, and the late-pregnancy swab may be skipped.
Factors That Raise the Risk
Not every GBS-positive mother faces the same level of risk. Three situations make transmission more likely to result in newborn illness:
- Testing positive for GBS late in pregnancy, confirming the bacteria are present close to delivery.
- Developing a fever during labor, which can signal an infection already underway.
- Having 18 or more hours pass between your water breaking and birth, giving bacteria more time to reach the baby.
When one or more of these factors are present, the case for preventive treatment during labor becomes especially clear.
How Preventive Antibiotics Work During Labor
If you test positive, you’ll receive IV antibiotics once labor begins or your water breaks. The goal is to reduce the amount of bacteria in the birth canal so fewer reach the baby during delivery. This approach has been the single biggest reason early-onset GBS disease has dropped so sharply over the past two decades.
The antibiotics work best when given at least four hours before delivery, so timing matters. If labor moves quickly and you don’t receive the full course, your medical team will monitor your newborn more closely for any signs of infection in the hours after birth. For most women, the process is straightforward: an IV line is placed, antibiotics run every few hours during labor, and no additional treatment is needed after delivery.
If you have a penicillin allergy, alternatives are available. Your provider will typically ask about your allergy history early in pregnancy so the right option is ready when labor starts.
What a Positive Result Means for Your Birth
A positive GBS screening does not change where or how you deliver. You can still aim for a vaginal birth, and the antibiotics don’t require any changes to your birth plan beyond the IV line. The bacteria are not something you “caught” or did anything to cause, and colonization has no effect on your own health during pregnancy.
After birth, a baby born to a GBS-positive mother who received timely antibiotics is observed for a standard period, usually 12 to 24 hours, before discharge. If antibiotics were not given or were given less than four hours before delivery, the observation period may be longer. Signs the medical team watches for include temperature instability, difficulty breathing, poor feeding, and unusual fussiness in the first few days of life. Late-onset disease, which appears after the first week, is not prevented by labor antibiotics, so knowing the symptoms remains important in the weeks following birth.