Late in the third trimester of pregnancy, most women are offered a routine test for Group B Streptococcus (GBS)—a type of bacteria that naturally lives in the digestive and genital tracts of many healthy adults. For most women, GBS causes no symptoms and poses no health risk. During pregnancy, however, understanding whether GBS is present becomes important because of the small but significant risk it can pose to a newborn during labor and delivery.
For women pregnant after 35, GBS testing is part of the standard prenatal care protocol just as it is at any age. Understanding what the test involves, what a positive result means, and how it’s managed can help you feel more informed as you approach the later weeks of pregnancy.
What Research Shows About GBS Prevalence in Pregnancy
According to the Centers for Disease Control and Prevention (CDC), approximately 1 in 4 pregnant women carry GBS bacteria. Carriage is normal—it doesn’t mean infection—and the status can change over the course of pregnancy, which is why testing is done close to the time of delivery, typically between 36 and 38 weeks gestation.
While GBS rarely causes illness in adults, it can be transmitted to a newborn during vaginal birth. In a small percentage of GBS-positive pregnancies where antibiotics aren’t given during labor, the baby may develop what’s called early-onset GBS disease, which can lead to serious illness. The rate of early-onset GBS disease has declined significantly since universal screening and intrapartum antibiotic prophylaxis became standard practice.
How GBS Testing Is Done
The GBS screening test involves a simple swab of the vaginal-rectal area, usually performed during a routine prenatal appointment between 36 and 38 weeks. The swab is sent to a laboratory where GBS bacteria are cultured. Results are typically available within a few days.
Some hospitals and birth centers offer rapid GBS testing at the time of admission in labor for women who didn’t have prenatal screening or whose status is unknown. This allows for faster decision-making about antibiotic use during labor, though the rapid test has a slightly lower sensitivity than standard culture.
What a Positive Result Means—and Doesn’t Mean
A positive GBS result indicates that GBS bacteria are present in the vaginal-rectal area at the time of testing. It does not mean you have an infection, nor does it mean your baby will definitely be affected. It means that intrapartum antibiotic prophylaxis—antibiotics given through an IV during labor—is recommended to reduce the risk of GBS transmission to the baby during delivery.
The standard antibiotic used for this purpose is penicillin, with alternatives available for women with penicillin allergies. Antibiotics are given during labor, not before—this is because treating GBS before labor doesn’t reliably prevent recolonization by the time of delivery. The goal is to reduce the bacteria present in the birth canal during the delivery process itself.
GBS and Cesarean Births
For women who are GBS-positive and have a planned cesarean delivery before labor begins and before the membranes rupture, routine GBS prophylaxis antibiotics may not be needed, since the baby is not passing through the birth canal. However, your care team will make this determination based on your individual situation and delivery circumstances. If labor begins before a planned cesarean or if membranes rupture, the calculus changes. Discussing your GBS results and birth plan with your OB/GYN or midwife well before your due date is always a good idea. Understanding the overall landscape of third-trimester prenatal care can help put these details in perspective.
Managing GBS in Preterm Labor
Premature labor presents additional considerations for GBS management, since testing earlier in pregnancy may not reflect GBS status at the time of delivery. In situations involving preterm labor before 36 weeks, GBS prophylaxis is often given based on risk factors and clinical circumstances rather than a culture result, since prenatal screening would not yet have been done. Your obstetric care team will follow current guidelines for GBS management in this context.
GBS Testing for Women With Prior GBS-Positive Pregnancies
A positive GBS test result in a previous pregnancy does not mean you’ll be positive in subsequent pregnancies—GBS colonization status can change. However, ACOG recommends that women who had a previous infant with GBS disease receive prophylaxis in all future pregnancies, regardless of screening results. Your OB/GYN will review your history and discuss how prior pregnancy outcomes factor into current recommendations. This is also a good example of why keeping your healthcare provider informed about your pregnancy history matters.
Frequently Asked Questions
Can I reduce my risk of testing GBS-positive?
GBS carriage is a natural phenomenon that is not caused by hygiene practices or lifestyle behaviors, and there are no established methods for reliably preventing GBS colonization. Research into GBS vaccines is ongoing. The most effective current strategy remains universal screening and intrapartum antibiotics when indicated.
Are the antibiotics given during labor safe for the baby?
Intrapartum antibiotics for GBS prophylaxis have been used routinely for decades and are considered safe. As with any antibiotic use, there are considerations around potential effects on the newborn’s microbiome, which researchers are continuing to study. Your care team can discuss the risk-benefit balance in your specific situation.
What if I’m allergic to penicillin?
Alternative antibiotics are available for women with penicillin allergies. The specific alternative depends on your allergy history and the results of GBS sensitivity testing. Informing your prenatal care team of any drug allergies early in pregnancy allows time to plan appropriately.
Can I deliver vaginally if I test positive for GBS?
Yes. A positive GBS result does not mean a vaginal delivery is contraindicated. The recommended management is intrapartum antibiotics to reduce transmission risk during vaginal delivery. Most GBS-positive women who receive appropriate antibiotic prophylaxis have uncomplicated deliveries.
Key Takeaways
- GBS is a common bacterium carried by approximately 1 in 4 pregnant women; carriage is normal and does not indicate infection.
- Routine screening between 36 and 38 weeks gestation is standard practice and guides decisions about intrapartum antibiotics during labor.
- A positive result means intrapartum antibiotic prophylaxis (typically penicillin IV during labor) is recommended to reduce—though not eliminate—the risk of GBS transmission to the newborn.
- GBS carriage status can change between pregnancies, so testing is recommended each time regardless of prior results.
- Discussing your GBS status and how it fits into your birth plan with your OB/GYN or midwife well before your due date allows time for informed planning.
Medical Disclaimer
This content is for informational purposes only and does not constitute medical advice. Individual health situations vary significantly. Always consult a qualified healthcare provider before making decisions related to your health, fertility, or pregnancy.
About the Author
Emily Carter is a women’s health writer focused on fertility, pregnancy after 35, and sleep changes in midlife. She writes research-informed, non-alarmist content to help women navigate reproductive and hormonal transitions with clarity and confidence.