The Group B strep test is a routine prenatal screening that checks whether you carry a type of bacteria called Group B Streptococcus (GBS) in your vaginal and rectal area. It’s done during the 36th or 37th week of pregnancy, and the result determines whether you’ll receive antibiotics during labor to protect your baby. The test itself is quick, painless, and takes only a few seconds.
Why the Test Matters
Group B strep is a normal bacterium that lives in the urogenital tract and rectum. It’s not a sexually transmitted infection, and carrying it doesn’t mean you’re sick. Somewhere between 15% and 40% of pregnant women are colonized with GBS at any given time, depending on geographic region. Most of the time, these bacteria cause no symptoms at all.
The concern is what happens during delivery. As a baby passes through the birth canal, GBS can transfer from mother to newborn. In a small number of cases, this leads to a serious blood infection, pneumonia, or meningitis in the first days of life. This is called early-onset GBS disease, and it typically develops within the first 48 to 72 hours after birth. Since the introduction of routine screening and preventive antibiotics in 1996, the rate of early-onset GBS disease has dropped significantly.
How the Test Works
The procedure is simple. Your provider uses a sterile swab (similar to a cotton swab) to collect a sample from your vagina and then your rectum. Both sites are swabbed because GBS can colonize either area. You may be given the option to collect the swab yourself, which studies have shown is equally accurate.
The swab is sent to a lab, where it’s placed in a culture medium that encourages GBS bacteria to grow. Results typically come back within 24 to 48 hours. A positive result means GBS was detected. A negative result means it wasn’t found at the time of testing.
Timing matters. The test is done at 36 to 37 weeks because GBS colonization can come and go. A result from earlier in pregnancy wouldn’t reliably predict your status at delivery. If you go past your expected due date, the result from your 36- to 37-week test is generally still used, though your provider may retest in certain situations.
What a Positive Result Means
A positive GBS test does not mean your baby will get sick. It means you carry the bacteria and there’s a risk of passing it during a vaginal delivery. To reduce that risk, you’ll receive antibiotics through an IV during labor. The goal is to have the antibiotics working in your system for at least four hours before delivery, which gives them enough time to significantly reduce the amount of bacteria in the birth canal.
Penicillin is the first-choice antibiotic for GBS prevention. If you have a penicillin allergy, your treatment depends on the severity of that allergy. A mild allergy (like a rash without breathing problems or swelling) typically means you can safely receive a closely related antibiotic called cefazolin. If your allergy history includes serious reactions like difficulty breathing, throat swelling, or hives, your provider will need to test the GBS bacteria collected during your screening to see which alternative antibiotics will work. Resistance to some alternatives is increasing, so this susceptibility testing is an important step.
You won’t take antibiotics before labor begins, and you won’t take them at home. The antibiotics are only given intravenously once active labor starts or your water breaks.
When the Test Isn’t Needed
In a few situations, you’ll automatically receive antibiotics during labor regardless of a screening result. If GBS was found in your urine at any point during the current pregnancy, that indicates a high level of colonization, and you’ll be treated during labor without a separate swab test. The same applies if you previously had a baby who developed a GBS infection. In both cases, the risk is considered high enough that screening would be redundant.
If you arrive in labor and your GBS status is unknown (for example, if you didn’t have prenatal care or delivered before being tested), your provider will make a decision based on risk factors like preterm labor, prolonged rupture of membranes, or fever during labor.
What a Negative Result Means
A negative result means GBS wasn’t detected at the time of your test, and you won’t need antibiotics during labor specifically for GBS prevention. Keep in mind that colonization can fluctuate. A negative result in one pregnancy doesn’t carry over to the next. You’ll be screened again during every subsequent pregnancy at 36 to 37 weeks.
What Happens for the Baby
If you test positive and receive antibiotics for at least four hours before delivery, the risk of your baby developing early-onset GBS disease drops substantially. After birth, the medical team will monitor your baby for any signs of infection, such as difficulty breathing, feeding problems, or temperature instability. In most cases, babies born to GBS-positive mothers who received timely antibiotics do very well and don’t need any additional treatment.
If antibiotics weren’t given or were started less than four hours before delivery, the baby may be monitored more closely, sometimes for 24 to 48 hours. The specific observation protocol depends on how the baby looks and whether any risk factors are present.
It’s worth noting that labor antibiotics protect against early-onset disease, the type that occurs in the first week of life. Late-onset GBS disease, which can develop from one week to a few months after birth, is not prevented by antibiotics given during labor. Late-onset disease is much less common, and its causes are less well understood.