Group B Strep (GBS) Test
Group B Streptococcus (GBS) is a bacteria naturally present in the vagina and rectum of about 25% of women. It's harmless to adults but can cause serious infection in newborns if transmitted during birth.
Typically performed: Weeks 35–37
How the Test Is Done
The GBS swab is a simple, quick procedure done between 35–37 weeks. A cotton swab is used to take samples from the vagina and rectum. Results come back within 24–48 hours. It does not require any preparation and is minimally uncomfortable.
If You Test GBS Positive
Testing GBS positive doesn't mean you're sick — it's a normal state of colonization. When you arrive in labor, you'll receive IV antibiotics (typically penicillin) every 4 hours until delivery. This reduces the risk of neonatal GBS infection from 1 in 200 to less than 1 in 4,000.
Special Situations
If you have GBS bacteriuria (GBS found in a urine culture) earlier in pregnancy, you are automatically treated with antibiotics in labor regardless of the late-pregnancy swab. If you go into labor or your water breaks before the test is done, your provider will make a decision based on risk factors.
Frequently Asked Questions
Without treatment, GBS-positive mothers have a 1 in 200 chance of their baby developing early-onset GBS disease, which can cause pneumonia, sepsis, or meningitis. With IV antibiotics in labor, this risk drops to less than 1 in 4,000.
There's no reliable way to prevent GBS colonization. It's a naturally occurring bacteria and colonization status can change. Some research suggests probiotics may help, but evidence is limited.
Babies born by C-section with intact membranes (water hasn't broken) are at very low risk of GBS transmission, so routine antibiotics specifically for GBS may not be given. However, all C-sections receive prophylactic antibiotics for surgical site prevention.