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Strep B Test
Reviewed by Medicinsk Expert•Last updated: 2026-10-22•Lab Tests

Strep B Test

A group B strep (GBS) test screens for bacteria that are generally harmless to adults but can cause serious, life-threatening infections in newborns.

What is a group B strep test?

Group B strep (GBS) is a type of bacteria commonly found in the digestive, urinary, and genital tracts. While usually harmless, it can cause GBS disease in older adults, people with chronic health conditions, or those with weakened immune systems. In pregnant individuals, GBS can be passed to the baby during vaginal delivery, potentially causing serious illness in the newborn.

GBS Infections in Newborns

  • Early-onset: Symptoms start within the first six days of life.
  • Late-onset: Symptoms start between 7 and 89 days of life.

In newborns, GBS can lead to bacteremia, pneumonia, and meningitis.

What is it used for?

The test is primarily used for routine prenatal screening to identify GBS bacteria. It may also be used to test infants who show signs of infection.

Why do I need a group B strep test?

Testing is recommended for all pregnant individuals, typically between 36 and 38 weeks of pregnancy. If you go into labor earlier, you may be tested at that time. While pregnant individuals usually have no symptoms, newborns with GBS may show signs such as fever, trouble feeding, breathing difficulties, lethargy, or irritability.

What happens during a group B strep test?

For pregnant individuals, the provider will use a cotton swab to collect cells and fluids from the vagina and rectum. A urine test may also be used. If a baby requires testing, a provider may perform a blood test (via heel prick) or a lumbar puncture (spinal tap) to collect cerebrospinal fluid.

Will I need to do anything to prepare for the test?

There are no special preparations required for a group B strep test.

Are there any risks to the test?

There is no risk to the pregnant individual from a swab or urine test. Infants may experience slight pain or bruising from a blood test or spinal tap. There is a small risk of infection or bleeding associated with a lumbar puncture.

What do the results mean?

If you test positive for GBS during pregnancy, you will be given intravenous (IV) antibiotics during labor to prevent passing the bacteria to your baby. If a baby tests positive for a GBS infection, they will be treated with antibiotics. If a provider suspects an infection, they may begin treatment before test results are available due to the potential severity of the disease.

Frequently Asked Questions

positive result does not guarantee your baby will get infected, but it means you will receive an intravenous antibiotic (usually penicillin or ampicillin) during labor to dramatically lower the risk of neonatal infection.
While antibiotics given during labor reduce the risk by about 80–90 %, a small percentage of babies can still develop early‑onset GBS disease. Your provider will monitor the newborn closely for any signs of infection.
The standard recommendation is to test between 36 and 38 weeks of gestation. If you go into labor earlier, your provider may test you at that time to determine if you need antibiotics during delivery.
Yes, a test can be performed on the newborn if the baby shows signs of infection, but it is not a substitute for prenatal screening. The benefit of post‑delivery testing is limited because the window for effective antibiotic prophylaxis has passed.
Source: MedlinePlus, National Library of Medicine

MedlinePlus brings together authoritative health information from the National Library of Medicine (NLM), the National Institutes of Health (NIH), and other government agencies and health-related organizations.

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