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The Group B Strep Test, Explained

By Erica Patino
Reviewed by Alyssa Quimby, M.D.
November 20, 2023

The group B strep test is a routine screening during pregnancy. It’s done around 35 to 37 weeks. The test is looking for group B strep (GBS), a type of bacteria that is mostly found in a woman’s vagina and rectum.

Group B strep is common and is harmless to you. “But it can be dangerous to a newborn,” says Kimberly Langdon, M.D., a board-certified ob-gyn with Medzino based in Ohio. “If the test is positive, you’ll receive an antibiotic in labor to prevent transmission [to your baby].

It’s recommended that all pregnant people take this test. If GBS is untreated, the baby can contract it from you during birth. GBS can lead to long-term problems like meningitis, a serious infection, or it can very rarely be fatal.

What Happens in the Group B Strep Test?

Your healthcare provider checks your urine during your first trimester for many things, including GBS. “If we see GBS in the urine in the first trimester, we would treat your urine infection with antibiotics and then give you antibiotics during labor,” says Sunny Soroosh, a certified nurse-midwife at Lenox Hill Hospital in New York City.

If you don’t have GBS in early pregnancy, you’ll get another test toward the end of pregnancy. The doctor or midwife will use a cotton swab to take a sample of bacteria growing on the skin around your vagina and rectum.

What Happens If I Test Positive for Group B Strep?

If you test positive for group B strep, don’t worry. About 1 in 4 pregnant people do, according to the CDC. With treatment, you can prevent any problems for your baby.

If you plan to have a vaginal birth, you’ll be given intravenous (IV) antibiotics once you’re in labor. This prevents the bacteria from being passed to the baby. If you’re going to deliver by c-section, you won’t need antibiotics unless you go into labor before your scheduled c-section.

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