The most effective way to prevent Group B Strep (GBS) from harming your baby is a two-step process: getting screened late in pregnancy and receiving IV antibiotics during labor if you test positive. This approach has dramatically reduced early-onset GBS infections in newborns over the past two decades. There is currently no proven way to eliminate GBS colonization before delivery, so prevention centers on stopping transmission during birth itself.
Why GBS Matters During Delivery
GBS is a common bacterium that lives in the digestive and reproductive tracts of roughly 25% of healthy women. It usually causes no symptoms and isn’t a sign of poor hygiene or infection. The concern is specific to childbirth: as the baby passes through the birth canal, GBS can transfer to the newborn and cause serious illness, including bloodstream infections, pneumonia, and meningitis.
Most newborns who develop GBS disease show symptoms on the day they’re born. Between 4% and 6% of babies who develop GBS disease will die from it, and survivors of GBS meningitis face a higher risk of long-term complications like hearing loss and developmental disabilities. These numbers sound alarming, but the actual rate of infection is low when preventive antibiotics are given during labor.
The Screening Test
Universal screening is recommended between 36 and 37 weeks of pregnancy. Your provider will collect a swab from both the vagina and the rectum. The rectal sample is important because it catches colonization that a vaginal swab alone would miss, so both sites need to be tested together. Results typically come back within 24 to 48 hours.
A positive result means you’re carrying GBS at the time of the test. It doesn’t mean you’re sick or that anything has gone wrong. It simply flags you for antibiotics during labor. Because GBS colonization can come and go, the timing of the screen close to your due date gives the most accurate picture of your status when delivery actually happens.
Antibiotics During Labor
If you screen positive, you’ll receive antibiotics through an IV once labor begins. Penicillin is the first-choice medication. The goal is to get at least one full dose into your system before delivery, with the best protection coming when the antibiotics have been running for four or more hours. This gives the medication enough time to reach effective levels in the amniotic fluid and birth canal, reducing the number of bacteria your baby encounters on the way out.
The antibiotics are given during labor only, not in the weeks leading up to it. Taking oral antibiotics earlier in pregnancy does not prevent GBS from returning by the time you deliver, which is why the strategy focuses on the window that matters most.
If You Have a Penicillin Allergy
Your provider will assess how severe your allergy is. If your past reaction was mild (a rash, for example, with no breathing difficulty or swelling), a related antibiotic called cefazolin is typically used instead. If you’ve ever had a serious allergic reaction involving swelling, hives, or trouble breathing after penicillin or a similar drug, a different class of antibiotic is chosen based on lab testing of your specific GBS strain. When sensitivity results aren’t available by the time labor starts, vancomycin is the usual backup.
This is one reason it’s worth mentioning any drug allergies early in your prenatal care. It gives your provider time to request the right lab work on your GBS culture so the best alternative is ready when you need it.
When Your GBS Status Is Unknown
Sometimes labor starts before the screening window, or results aren’t back yet. In those situations, antibiotics are given based on risk factors present during labor. You’ll receive prophylactic antibiotics 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
Each of these conditions increases the chance that bacteria can travel upward into the uterus and reach the baby. Longer time with ruptured membranes, in particular, gives bacteria more opportunity for ascending infection. Fever during labor can signal an inflammatory response to bacterial infection already underway.
Other Factors That Raise Risk
Two situations automatically qualify you for antibiotics in labor regardless of your current culture results. If you had GBS bacteria detected in your urine at any point during this pregnancy, that signals a high level of colonization and greater risk to the baby. And if you previously delivered a baby who developed GBS disease, the risk is considered elevated for all future pregnancies.
Some labor practices may also play a role. Observational studies have linked frequent vaginal exams during labor, invasive fetal monitoring, and membrane sweeping with higher rates of early-onset GBS disease, likely because these procedures can introduce bacteria further into the birth canal. This doesn’t mean these interventions should always be avoided, but it’s a factor providers weigh when managing labor in GBS-positive patients.
Do Probiotics or Vaginal Washes Work?
Many women look for natural ways to clear GBS before delivery. The evidence so far is not encouraging. A randomized controlled trial tested a daily oral probiotic (containing multiple bacterial strains) taken from 28 weeks until labor. GBS colonization dropped by only 5% compared to placebo, a difference so small it was statistically meaningless. There was also no reduction in the number of antibiotic doses women needed during labor.
Vaginal washes with chlorhexidine, an antiseptic liquid, have also been studied. A Cochrane review of the available trials found no reduction in early-onset GBS disease, GBS pneumonia, or GBS-related deaths in newborns. The washes did, however, increase the rate of mild side effects in mothers. Based on current evidence, neither probiotics nor chlorhexidine vaginal rinses are recommended as alternatives to IV antibiotics during labor.
What You Can Do Before Delivery
The most practical steps are straightforward. Make sure your provider orders the GBS screen at the right time, between 36 and 37 weeks. Confirm that your birth plan accounts for IV antibiotics if needed, especially if you’re planning a home birth or birth center delivery where IV access may require advance coordination. Let your provider know about any antibiotic allergies as early as possible so alternative medications can be arranged.
If you go into labor before your screening appointment, or if your water breaks early, head to your delivery location promptly. The sooner antibiotics can start, the more time they have to work before your baby arrives. Letting your care team know your GBS status (or that it’s unknown) when you check in helps them act quickly.