Group B Strep in Pregnancy

Group B strep (GBS) is a common bacterium that lives harmlessly in the gut, rectum, and vagina of many healthy adults. It is not a sexually transmitted infection, and carrying it is not a sign you did anything wrong or that you're unwell. The CDC estimates about 1 in 4 pregnant people carry GBS. Carriage comes and goes over time, which is why the swab is done close to your due date rather than earlier. GBS matters in pregnancy for one reason: rarely, it can pass to a baby around birth and make the newborn sick — and that's very preventable with antibiotics in labor.

ACOG recommends screening every pregnant person at 36 0/7 to 37 6/7 weeks — a shift from the older 35–37 week window. The test is a single, quick swab of the lower vagina and rectum, which you or your clinician can do; it isn't painful and takes seconds. The lab grows a culture over a couple of days, so results usually come back within about 48–72 hours. Because GBS carriage changes over time, this late-pregnancy swab is the one used to guide whether you're offered antibiotics in labor. Testing positive is common and expected — roughly a quarter of results are positive.

A positive swab means you'll be offered IV antibiotics — usually penicillin — during labor, started when labor begins or your water breaks and repeated at intervals until birth. The goal is narrow and specific: to prevent early-onset GBS disease in the newborn, which the antibiotics reduce dramatically. It does not mean you're sick, that you need antibiotics before labor, or that a C-section is required — most people with GBS have an uncomplicated vaginal birth. It also doesn't mean you'll always carry GBS; it's about protecting the baby during those hours around delivery.

It changes which antibiotic is used, not whether you get one. Your care team chooses the alternative based on how severe the allergy is: for a milder reaction like a rash, a closely related antibiotic (cefazolin) is typical; for a severe reaction such as anaphylaxis, swelling, or trouble breathing, the options are clindamycin — but only if lab testing shows your specific GBS is susceptible to it — or otherwise vancomycin. Which one is right is a clinical decision made by your team from your history and the lab results, not something to choose yourself. Tell them exactly what reaction you had so they can pick the safest option.

If labor starts before your swab is done or before the result is back — or comes very quickly — your team uses a risk-based approach instead: antibiotics are offered if you're preterm (before 37 weeks), your water has been broken for 18 hours or more, or you develop a fever in labor. If you're planning a home birth or water birth, talk with your midwife or doctor ahead of time about how IV antibiotics in labor would be handled in that setting, since it affects timing and monitoring. This is an important conversation to have well before your due date, not something to sort out in the moment.

Yes. This tool runs entirely in your browser. Your answers are used on your own device and are never sent to a server, saved, or shared — close the tab and they're gone. In a post-Dobbs world we treat reproductive and pregnancy data as sensitive by default. It's general educational information based on ACOG and CDC guidance; it can't diagnose you, isn't a substitute for your own prenatal care, and any decisions about testing and antibiotics are made with your clinician.