Preventing Group B Strep (GBS) complications in pregnancy comes down to two steps: getting screened at the right time and receiving antibiotics during labor if you test positive. About 1 in 5 pregnant women carry GBS bacteria naturally, and while it’s harmless to you, it can cause serious infection in a newborn during delivery. The good news is that the current prevention strategy has reduced early-onset GBS disease in newborns by more than 80% since the early 1990s.
What GBS Colonization Actually Means
GBS is a type of bacteria that lives in the digestive and genital tracts of many healthy adults. Roughly 18% of pregnant women worldwide carry it at any given time, with rates varying by region from about 11% to 35%. Carrying GBS doesn’t mean you have an infection. It doesn’t cause symptoms, and it isn’t a sign of poor hygiene or a sexually transmitted condition. The bacteria can come and go on their own, which is why testing earlier in pregnancy isn’t reliable enough to predict your status at delivery.
The risk arises during labor and delivery, when GBS can pass to the baby. Without preventive treatment, a small percentage of babies born to colonized mothers develop early-onset GBS disease, which can cause bloodstream infections, pneumonia, or meningitis in the first week of life.
When and How You’re Screened
The standard recommendation from the American College of Obstetricians and Gynecologists is universal screening during the 36th or 37th week of each pregnancy. Your provider uses a swab (similar to a Q-tip) to collect a sample from both the vagina and rectum. Results typically come back within a day or two.
This screening happens with every pregnancy, even if you tested negative before. Your colonization status can change between pregnancies, so previous results don’t count. If GBS is found in your urine at any point during pregnancy, that alone confirms you’re colonized, and you won’t need the swab test later. High levels of GBS in urine also require treatment during pregnancy itself, not just during labor.
IV Antibiotics During Labor
The primary prevention method is receiving antibiotics through an IV once labor begins. This is called intrapartum antibiotic prophylaxis, and it’s the single most effective tool for protecting your baby. Timing matters: antibiotics need at least four hours in your system before delivery to work best. When that timing window is met, effectiveness against early-onset GBS disease reaches about 91% for full-term babies and 86% for preterm babies.
Shorter windows are significantly less protective. When antibiotics are given for less than two to four hours before delivery, effectiveness drops to roughly 38 to 47%. That’s why your care team will start the IV as early in labor as possible once you’re admitted.
You’ll receive doses every four hours until your baby is born. Most women tolerate this well, and it doesn’t restrict your movement during labor or change your birth plan in any major way beyond having an IV line.
If You Have a Penicillin Allergy
The first-line treatment is penicillin, with ampicillin as an alternative. If you’re allergic to penicillin but have never had a severe reaction (like anaphylaxis or throat swelling), you’ll likely receive a related antibiotic called cefazolin, which is safe for people with mild penicillin allergies.
If you have a history of severe allergic reactions, your provider will test whether your GBS bacteria respond to clindamycin. If the bacteria are susceptible, clindamycin is used. If not, vancomycin is the backup. It’s worth noting that clindamycin-based prevention has shown lower effectiveness (around 22%) compared to the standard approach, so knowing your allergy history in advance and discussing it with your provider well before labor gives your team the best chance to plan effectively.
Risk Factors That Increase Transmission
Certain labor conditions raise the chance of passing GBS to the baby, even among colonized women. The most significant ones are:
- Prolonged rupture of membranes. When your water breaks 18 or more hours before delivery, the risk of vertical transmission nearly doubles.
- Premature rupture of membranes at term. Water breaking before labor starts (even at full term) also increases risk, roughly doubling the likelihood of transmission.
- Fever during labor. A maternal temperature at or above 37.5°C (about 99.5°F) during labor is associated with a 40% increase in transmission risk.
These factors are why women with certain risk factors may receive antibiotics during labor even without a confirmed positive GBS test, particularly if screening results aren’t available in time.
Planned Cesarean Delivery
If you’re GBS-positive and scheduled for a cesarean section before labor begins and before your water breaks, the risk of transmitting GBS to your baby is extremely low. In this scenario, intrapartum GBS antibiotics are generally not needed because the baby doesn’t pass through the birth canal where the bacteria reside. You’ll still receive the standard pre-surgical antibiotics that are given before any cesarean to prevent wound infections. However, if labor starts unexpectedly or your membranes rupture before the surgery, GBS prophylaxis applies just as it would for a vaginal delivery.
Can Probiotics Reduce GBS Colonization?
Some women look for natural approaches to clear GBS before delivery. One randomized controlled trial tested oral probiotics containing two specific Lactobacillus strains in 110 GBS-positive pregnant women. Among those taking the probiotic, 42.9% converted from GBS-positive to negative, compared to 18% in the placebo group. That’s a statistically meaningful difference.
However, a single trial with 110 participants isn’t strong enough evidence to replace the standard antibiotic approach. Converting to negative on a follow-up test also doesn’t guarantee you’ll stay negative through delivery. Probiotics are generally considered safe during pregnancy, but they should be viewed as a complement to, not a substitute for, screening and antibiotics during labor. If you test positive at 36 to 37 weeks, IV antibiotics during labor remain the proven strategy for protecting your baby.
What You Can Do Before Labor
There’s no reliable way to prevent GBS colonization itself. It’s a normal part of the body’s bacterial landscape, and no amount of dietary changes, supplements, or hygiene practices can guarantee you won’t carry it. What you can control is being prepared. Make sure your GBS screening happens during weeks 36 or 37, confirm the results are in your medical record, and communicate your status (along with any drug allergies) to whoever will be present at your delivery. If you’re delivering at a different hospital or birth center than where you received prenatal care, bring a copy of your test results.
If your water breaks at home, note the time. That information helps your care team assess your risk window and prioritize getting antibiotics started quickly. Arriving at the hospital promptly after your water breaks or labor begins gives the best chance of hitting that critical four-hour antibiotic window before delivery.