Group B Streptococcus (GBS) screening is a routine part of late-pregnancy care, typically performed between 35 and 37 weeks of gestation. For many women—particularly those pregnant for the first time after 35—the test and the conversations around it can feel unfamiliar. Understanding what GBS is, what a positive result means, and how it is managed can help reduce anxiety and support clearer communication with a healthcare team.
According to the American College of Obstetricians and Gynecologists (ACOG), Group B Streptococcus is a bacterium commonly found in the gastrointestinal and genital tracts of healthy adults. Approximately 10–30% of pregnant women carry GBS without symptoms. While GBS colonisation is generally harmless for the carrier, it can cause serious infections in newborns if transmitted during delivery.
What Group B Strep Is and Why It Matters in Pregnancy
GBS is not a sexually transmitted infection—it is a bacterium that naturally colonises the gut and lower genital tract in many healthy adults. Carrying GBS during pregnancy does not cause illness in the mother and is not related to hygiene or health choices. Colonisation status can change over time, which is why testing is performed late in pregnancy rather than earlier.
The concern with GBS in pregnancy centres on the potential for transmission to a newborn during labour and delivery. A small proportion of newborns exposed to GBS develop early-onset GBS disease—a serious condition that can involve sepsis, pneumonia, or meningitis. Routine screening and, when indicated, intrapartum antibiotic prophylaxis (antibiotics given during labour) have significantly reduced the incidence of early-onset GBS disease in newborns over the past few decades.
How GBS Screening Works
GBS screening involves a simple swab of the lower vagina and rectum, typically performed between 36 and 37 weeks of pregnancy (some guidelines recommend 35–37 weeks). The swab is sent to a laboratory for culture, with results usually available within 24–48 hours. The test is brief and can be performed during a routine prenatal appointment.
For women who have had a previous baby with GBS disease, or who have GBS detected in urine during the current pregnancy, prophylactic antibiotics during labour are recommended regardless of a late-pregnancy swab result. For context on the broader prenatal testing landscape, our overview of prenatal testing after 35 covers what other screenings to expect.
What a Positive GBS Result Means
A positive GBS screening result means that GBS bacteria were detected in the culture at the time of testing. It does not indicate an infection, illness, or cause for immediate concern. The standard recommendation for GBS-positive women is intrapartum antibiotic prophylaxis—antibiotics (most commonly intravenous penicillin or ampicillin) administered during labour to reduce the risk of transmission to the baby.
The decision to administer antibiotics is typically straightforward for GBS-positive women, but women with penicillin allergy may require alternative antibiotic regimens; informing your healthcare team of any allergies is important so the appropriate protocol can be planned in advance.
GBS, Planned Caesarean, and Preterm Labour
For women with a planned caesarean delivery who go into labour or have their membranes rupture before the surgery, intrapartum antibiotics for GBS are still recommended if the woman is GBS positive. For women who deliver by elective caesarean with intact membranes and no labour, the recommendation is different—your obstetric team will advise on the specific protocol for your situation.
Women who go into labour before 37 weeks and have not yet had GBS testing should be treated as GBS-positive for prophylaxis purposes, given that testing has not been performed and early delivery increases neonatal risk. This is a standard protocol that your healthcare team will manage.
Frequently Asked Questions
Does being GBS positive mean my baby will definitely be affected?
No. The majority of babies born to GBS-positive mothers do not develop GBS disease, particularly when intrapartum antibiotics are administered as recommended. Research shows that prophylactic antibiotics significantly reduce the already-low risk of early-onset GBS disease in newborns. Your healthcare team will ensure appropriate protocols are in place for your labour and delivery.
Can GBS status change between pregnancies?
Yes. GBS colonisation status is not permanent and can change over time. A woman who was GBS positive in a previous pregnancy may test negative in a subsequent one, and vice versa. This is why screening is repeated in each pregnancy rather than relying on results from a previous pregnancy.
Are there risks to receiving antibiotics in labour?
Intrapartum penicillin or ampicillin is generally well tolerated. Minor side effects can include mild nausea or rash. Serious allergic reactions are rare. For women with a documented penicillin allergy, alternative antibiotics are available; informing your care team of any allergies well in advance allows appropriate planning. The benefits of prophylaxis in GBS-positive women are considered to outweigh the risks of antibiotic use.
Is GBS more common in women over 35?
GBS colonisation does not appear to be significantly more prevalent in women over 35 compared to younger pregnant women. The bacteria can be present in women of any age. The recommendation for routine screening applies to all pregnant women regardless of age, and management is the same across age groups.
Key Takeaways
- GBS screening is routine at 36–37 weeks of pregnancy; a positive result is common (affecting 10–30% of pregnant women) and does not indicate illness.
- A positive GBS result means intrapartum antibiotics will be recommended during labour to reduce the risk of transmission to the newborn.
- Antibiotics during labour are highly effective at preventing early-onset GBS disease in newborns and are well tolerated by most women.
- GBS colonisation status can change between pregnancies, so testing is repeated in each pregnancy.
- Informing your healthcare team of any penicillin allergy in advance allows appropriate antibiotic planning if GBS prophylaxis is needed.
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.