GUIDE
The GBS test is a simple swab at 36-37 weeks that checks if you carry group B streptococcus bacteria. About 25% of people test positive.
GBS is a normal bacterium that does not cause illness in healthy adults. However, it can be passed to the baby during vaginal delivery and cause serious infection in newborns. IV antibiotics during labor effectively prevent transmission.
Be prepared from day one
Group B streptococcus (GBS, or Streptococcus agalactiae) is a type of bacteria that commonly lives in the gastrointestinal and genital tracts of healthy adults. It is not a sexually transmitted infection and is not related to group A strep (which causes strep throat). Carrying GBS is completely normal and does not mean you are sick, unclean, or have done anything wrong.
The concern with GBS in pregnancy is specific: during a vaginal delivery, the bacteria can be passed to the baby as it moves through the birth canal. In rare cases (about 1 to 2 per 1,000 births without treatment), GBS can cause serious neonatal infections including sepsis, pneumonia, and meningitis. IV antibiotics during labor reduce this risk by approximately 80 percent.
This is why ACOG and the CDC recommend universal GBS screening for all pregnant people at 36 to 37 weeks, as part of the standard prenatal visit schedule.
GBS colonization is extremely common and not a sign of any health problem.
The GBS test is a simple swab — not a blood test. Your provider (or you, in some practices) swabs the lower vagina and the rectum. The swab is sent to a lab where the bacteria are cultured. Results are typically available in 24 to 48 hours.
The test is done at 36-37 weeks because GBS colonization can come and go. A test done earlier in pregnancy would not reliably predict your status at the time of delivery. If you tested positive for GBS in a previous pregnancy, you will still be tested again this time — your status may have changed.
There is one exception: if GBS was found in your urine (GBS bacteriuria) at any point during this pregnancy, you are automatically treated as GBS-positive and will receive antibiotics during labor regardless of the swab result.
Based on CDC/ACOG 2020 prevention guidelines. Your provider will determine whether antibiotics are indicated.
If you test positive for GBS, you will receive IV antibiotics when you arrive at the hospital in active labor or when your water breaks — whichever comes first. The antibiotics work by reducing the number of GBS bacteria in the birth canal before the baby passes through.
Ideally, you should receive at least two doses of antibiotics (given 4 hours apart) before delivery for optimal protection. This is one reason why going to the hospital promptly when labor begins is important if you are GBS-positive.
The antibiotics are given through an IV line and do not restrict your movement significantly. You can still walk, use a birthing ball, and change positions during labor.
| Antibiotic | Details |
|---|---|
| Penicillin G | First-line treatment. Given IV every 4 hours during labor until delivery. |
| Ampicillin | Alternative first-line option. Same dosing schedule as penicillin. |
| Cefazolin | Used for patients with a mild penicillin allergy (no anaphylaxis history). |
| Clindamycin or vancomycin | Used for patients with a severe penicillin allergy. Sensitivity testing required. |
If you are GBS-positive, include this information in your birth plan and make sure it is in your hospital chart. When you arrive in labor, remind the admitting team of your GBS status so antibiotics can be started promptly. The sooner treatment begins, the more effective it is.
If you received adequate antibiotic prophylaxis (at least 4 hours of IV antibiotics before delivery), your baby will be monitored with standard newborn care. No additional testing is typically needed for the baby.
If antibiotics were not given or were given less than 4 hours before delivery, the baby will be monitored more closely for signs of GBS infection during the first 24 to 48 hours. This may include observation for temperature instability, breathing issues, poor feeding, or lethargy. Blood cultures may be drawn if any symptoms develop.
Early-onset GBS disease (occurring within the first week of life) is the primary concern. Thanks to universal screening and antibiotic prophylaxis, early-onset GBS disease has decreased by about 80 percent since the 1990s.
For context on all the tests and checks happening in your third trimester, see our prenatal visit schedule. For more on blood work that happens alongside the GBS test, see our pregnancy blood tests guide. As you approach your due date, make sure your hospital bag is packed and review the signs of labor so you know when to head in. And for any medication questions during this final stretch, our pregnancy-safe medications guide is a helpful reference.
This guide is for informational purposes only and is not a substitute for professional medical advice. Always consult your healthcare provider with any questions about your pregnancy.
