Group B strep (GBS) is a common bacterium that lives in the vaginal and rectal areas of roughly 1 in 4 pregnant women. It’s usually harmless to the mother, but it can pass to the baby during delivery and, in rare cases, cause serious infection. That’s why prenatal screening and a simple preventive treatment during labor have become standard care.
What Group B Strep Actually Is
GBS is a type of bacteria that naturally comes and goes in the body. It can live in the digestive tract, vagina, and rectum without causing any symptoms. You can carry it for weeks or months, then test negative later. It’s not a sexually transmitted infection, it doesn’t mean you’re unclean, and it doesn’t indicate a problem with your pregnancy. A large international study published in The Lancet Microbe found that about 24% of pregnant women are colonized with GBS at any given time, though rates vary by region.
Being “colonized” simply means the bacteria are present. Most colonized women never feel sick from it, and most of their babies are perfectly fine. The concern is specifically about what can happen during vaginal delivery, when the baby passes through the birth canal and comes into contact with the bacteria.
How GBS Affects the Baby
About half of colonized mothers will pass GBS to their newborn during birth. That sounds alarming, but the vast majority of those babies simply become temporarily colonized themselves and never get sick. Without any preventive treatment, only 1 to 2% of exposed newborns develop an actual GBS infection, a condition called early-onset disease.
When infection does occur, it falls into two categories. Early-onset disease shows up within the first week of life, often within hours of birth, and can cause blood infection (sepsis), pneumonia, or, less commonly, meningitis. Late-onset disease appears between 7 and 89 days after birth and is more likely to cause meningitis. Early-onset disease is the type that prenatal screening and labor treatment are designed to prevent. Late-onset disease is less well understood and not as clearly linked to what happens during delivery.
These infections are serious when they do occur. But the combination of universal screening and antibiotics during labor has reduced early-onset GBS disease by about 80% in the United States since those measures became widespread.
How and When You’re Tested
Standard screening involves a simple swab of the vagina and rectum, typically done between 36 and 37 weeks of pregnancy. The swab is sent to a lab to check whether GBS is growing. Results usually come back within a day or two. There’s no preparation needed and the test itself takes only a few seconds.
The timing matters because GBS colonization can fluctuate. A test done earlier in pregnancy might not reflect your status at the time of delivery. Testing in the late third trimester gives the most accurate picture of whether the bacteria will be present when labor begins.
If GBS shows up in a urine culture at any point during pregnancy, that’s also treated as a positive result. You won’t need the late-pregnancy swab because GBS in the urine is considered a strong indicator that colonization levels are high enough to warrant treatment during labor.
What Happens If You Test Positive
A positive result doesn’t change anything about your pregnancy day to day. You won’t take antibiotics before labor, and you don’t need to do anything differently in the weeks leading up to delivery. The treatment happens during labor itself: you’ll receive IV antibiotics once contractions start or your water breaks. The goal is to reduce the amount of bacteria in the birth canal so the baby has less exposure during delivery.
Penicillin is the standard antibiotic used. If you have a penicillin allergy, your provider will choose an alternative based on the severity of your allergy. The antibiotics work best when given at least four hours before delivery, which is one reason your care team will want to know your GBS status early in labor. If labor progresses quickly and there isn’t a full four-hour window, you’ll still receive antibiotics, as any amount of treatment provides some protection.
If you’re having a planned cesarean delivery before labor starts and your water hasn’t broken, GBS antibiotics during the procedure are generally not needed. The baby bypasses the birth canal entirely in that scenario, so the risk of transmission is very low. However, if labor begins unexpectedly or your membranes rupture before the scheduled surgery, antibiotic treatment applies just as it would for a vaginal delivery.
What to Expect After Delivery
If you received antibiotics during labor for at least four hours, your baby will typically be monitored with routine newborn care. If antibiotics were given for a shorter window, or if there were other risk factors like a fever during labor or prolonged time between your water breaking and delivery, the medical team may observe the baby more closely for signs of infection in the first 24 to 48 hours.
Signs of GBS infection in a newborn include difficulty breathing, poor feeding, unusual fussiness or lethargy, and unstable body temperature. These symptoms can overlap with other newborn conditions, so the hospital staff will be watching for them regardless of your GBS status. Most GBS-positive mothers who receive timely antibiotics bring home healthy babies without any complications.
Why GBS Can Come and Go
One thing that confuses many women is testing positive in one pregnancy but negative in another, or hearing about friends who tested negative at 36 weeks and then had a different result later. GBS colonization is transient. The bacteria can establish themselves, disappear, and return over weeks or months. This is normal and doesn’t reflect anything about your health habits or hygiene. It’s simply how this particular bacterium behaves in the human body. That’s why each pregnancy requires its own screening, and why the test is timed as close to delivery as practical.