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Group B Strep Test

Group B strep can be passed from mother to child during birth and cause life-threatening infection in the newborn. Find out about testing.

pregnant woman talking to her doctor

At some time in your ninth month of pregnancy, you should be tested for germs called group B streptococcus (GBS). Between 10–30 percent of women carry GBS germs in the digestive system, the vagina or rectal area.

Many adults may not even know they have GBS. But if passed to the baby during vaginal delivery, it can cause life-threatening infections.

What is the GBS test?

The GBS test is a prenatal test to spot the GBS germs. It is a simple, painless swab of the vagina and rectum. The swab is sent to a lab.

Why would I need a GBS test?

The GBS germs can affect the baby as it passes through the birth canal. Babies with GBS infection can get pneumonia, sepsis (infection of the blood) or meningitis (infection of the membranes surrounding the brain). For this reason, the American College of Obstetricians and Gynecologists advises that all women are tested for GBS between the 36th and 38th weeks of pregnancy.

What if I test positive?

Carrying GBS germs does not mean that you have a sexually transmitted disease or that you’ve done anything wrong. GBS can happen in any healthy woman.

If you test positive for GBS germs, you will be treated with antibiotics through an IV when you go into labor. Penicillin is most often used, so be sure to let your doctor know during a checkup if you’re allergic to penicillin or other drugs. It’s a good idea to remind the hospital staff.

You may also be given antibiotics during labor if you have not been tested (Or your GBS status is unknown) and you:

  • Go into preterm labor (before 37 weeks of gestation)
  • Reach full term, but your water breaks with delivery expected to be more than 18 hours away or your water breaks more than 18 hours before delivery
  • Have premature rupture of membranes (Your water breaks before labor and before you reach full term)
  • Have an unexplained fever during labor
  • Have previously had a child who had a GBS infection
  • Known GBS positive in a previous pregnancy
  • Have GBS in your urine at any time during your current pregnancy

Studies show it is not helpful to give antibiotics during pregnancy except in certain circumstances: 1) you are symptomatic or 2) you have a high level of bacteria present. The germs are not completely eliminated and have time to come back before you go into labor.

Newborn infants are watched closely for signs of an infection, especially if you are GBS-positive at some point in your pregnancy. Babies who show signs of infection within the first 48 hours are treated with antibiotics. Chances are small that you can pass the germs on to your baby if you are treated with antibiotics during labor.

In some cases, a baby may show signs of a GBS infection a week to a few months after birth. Babies who show signs of infection are treated with antibiotics.

By Ginny Greene, Contributing Editor

American College of Obstetricians and Gynecologists (ACOG). Chapter 8. Month 9. In: ACOG’s Your Pregnancy and Childbirth: Month to Month. 6th Edition. Washington, D.C.: ACOG; 2015.
The American College of Obstetricians and Gynecologists. Group B Strep and Pregnancy. Accessed January 3, 2020.
Centers for Disease Control and Prevention. Protect your baby from group B strep. Accessed January 3, 2020.
UpToDate. Patient information: Group B streptococcus and pregnancy. Accessed January 3, 2020.

Last Updated: January 3, 2020