Preventing Group B Strep (GBS) infection in newborns relies on a two-step approach: screening during pregnancy and receiving antibiotics during labor. This strategy has reduced early-onset GBS disease in newborns by roughly 80% since the early 1990s, making it one of the most effective infection-prevention measures in modern obstetrics.
GBS is a bacterium that naturally lives in the digestive and genital tracts of about 25% of healthy adults. It causes no problems for most people, but during childbirth it can pass to the baby and cause serious infections like sepsis, pneumonia, or meningitis. Here’s what actually prevents that from happening.
Prenatal Screening: The First Step
Universal screening is now standard prenatal care. Between 36 and 38 weeks of pregnancy, your provider will use a swab to collect a sample from the vagina and rectum. The sample is sent to a lab to see whether GBS bacteria are growing there. A positive result doesn’t mean you’re sick or that anything is wrong. It simply means the bacteria are present and your care team needs a plan for delivery.
Colonization can come and go, which is why the test is done late in pregnancy rather than earlier. A result from 20 weeks wouldn’t reliably predict what’s happening at delivery. If you’ve had a previous pregnancy where you tested positive for GBS, that history may also factor into your care plan if your current screening status is unknown when labor begins.
Antibiotics During Labor
If you screen positive, the primary prevention method is receiving IV antibiotics during labor, a strategy called intrapartum antibiotic prophylaxis (IAP). The antibiotics work by reducing the amount of GBS bacteria in the birth canal so the baby is far less likely to be exposed during delivery.
Timing matters. The antibiotics need at least 4 hours to reach effective levels before the baby is born. This is why your care team will want to start the IV relatively early in active labor. If your water breaks or you’re being induced, let your provider know your GBS status right away so there’s enough time.
For people with a penicillin allergy, alternative antibiotics are available. Your provider will assess the severity of your allergy to choose the safest option. The lab may also test your specific GBS strain to confirm which antibiotics will work, since resistance patterns vary.
What Happens If Your Status Is Unknown
Sometimes labor starts before screening results are back, or the test was never done. In these situations, antibiotics during labor are recommended if any of the following apply:
- Preterm labor: delivery before 37 weeks
- Prolonged membrane rupture: your water has been broken for 18 hours or more
- Fever during labor: a temperature of 100.4°F (38°C) or higher
Some hospitals also have rapid molecular tests that can detect GBS in about an hour, which can guide the decision when culture results aren’t available. If you had GBS in a previous pregnancy and your current status is unknown at 37 weeks or later, your provider will likely recommend antibiotics as a precaution.
What IAP Does Not Prevent
Antibiotics during labor are highly effective at preventing early-onset GBS disease, which occurs in the first week of life. But they do nothing to prevent late-onset disease, which appears between one week and three months after birth. Late-onset GBS can come from sources other than the birth canal, including contact transmission after delivery, so the timing of labor antibiotics simply can’t address it.
This is a significant gap. While early-onset cases have dropped dramatically, late-onset GBS rates have remained essentially unchanged for decades. Researchers are actively working on maternal vaccines that could protect against both types by passing antibodies to the baby during pregnancy. Several vaccine candidates are currently in clinical trials. If successful, a vaccine given during pregnancy could provide protection that lasts well beyond delivery, covering the window where late-onset disease strikes.
Probiotics and GBS Colonization
Some pregnant people wonder whether probiotics can reduce GBS colonization and potentially avoid the need for antibiotics during labor. A systematic review and meta-analysis of randomized controlled trials found that oral probiotic supplementation did modestly reduce vaginal GBS colonization. About 32% of women in the probiotic group tested positive at 35 to 37 weeks, compared to nearly 39% in the control group. Probiotics started after 30 weeks of gestation appeared to be more effective, and women who were already GBS-positive showed a meaningful rate of converting to negative after treatment.
These results are promising but not strong enough to replace standard screening and antibiotics. Probiotics reduced colonization, but they didn’t eliminate it. If you’re interested in trying probiotics alongside standard care, discuss it with your provider, but don’t skip the screening test or decline antibiotics based on probiotic use alone.
Room for Improvement
Even with current prevention strategies, GBS disease hasn’t been fully eliminated. Researchers estimate that under optimal implementation of existing guidelines, early-onset cases could be reduced by an additional 26 to 59%, with the single biggest gain coming from better use of antibiotics during labor. In practical terms, this means the system works well when every step goes as planned: screening happens on time, results are available, labor allows enough time for antibiotics, and the right antibiotic is chosen. Prevention gaps tend to occur when one of those steps is missed.
The most important thing you can do is make sure your GBS screening happens during the recommended window of 36 to 38 weeks, confirm the result is in your medical record, and communicate your status clearly when you arrive at the hospital in labor. If you’re planning a home birth or birth center delivery, discuss GBS prevention logistics with your midwife well in advance so the antibiotic plan is in place before labor starts.