Pregnancy Information Center

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Group B Strep: What Pregnant Women Need to Know


Learn about testing and treatment for the bacterium known as GBS

By Elaine Brown, MD

What Is Group B Streptococcus?

Group B streptococcus, also knows as group B strep or GBS, is a type of bacteria that typically colonizes the vagina and lower gastrointestinal (GI) tract of women. To determine whether a mother is a carrier of the bacterium, a swab test (from the vagina or lower GI tract) or urine culture is performed in all mothers-to-be between 35 to 37 weeks gestation; if GBS bacteria is detectable on the swabs or the urine culture is positive, this means the woman's system is colonized by GBS. It’s likely that virtually every woman's system is colonized by GBS at some point in her lifetime.

Approximately 20 to 30% of all pregnant women are carriers of the bacterium at any given point in time. Being a carrier is different from having an infection because a carrier has no symptoms, but the potential for infection is there. Also, a woman may test positive in one pregnancy but negative in the next. GBS isn’t considered a sexually transmitted infection.


Infections Caused by Group B Strep

In newborns, GBS infections (also known as perinatal group B streptococcal disease) are divided into two groups: early onset and late onset. 

Early onset cases occur within the first week of life, with most cases diagnosed on the day of the baby's birth or within 72 hours. Late onset cases occur after the first week of life. Meningitis (infection of the spinal fluid) is diagnosed in up to one-third of late onset cases and permanent neurologic injury can follow. GBS can cause infection in the blood, lungs, brain or spinal fluid of the baby, or it can result in milder infections, of the skin, for example

In mothers, infection with GBS can cause urinary tract infections or UTIs, infection of the placenta and membranes during labor, infection of the uterus after delivery or infection of the blood.

Prevention of these infections is the goal of the Centers for Disease Control and Prevention guidelines to universally screen pregnant women for GBS. Once you’re tested, if you’re determined to be a current carrier (meaning your test is positive), you’re given antibiotics while you’re in labor to prevent infections in you and your newborn. Women who’ve had a urinary tract infection caused by GBS during their pregnancy and mothers who’ve had a baby with GBS in the past are also treated. Women with preterm labor or whose water breaks preterm are also treated with antibiotics until their culture results are available (usually within 48 hours).

This universal testing approach has been quite successful — since the 1990s there has been approximately an 80% decrease in early-onset GBS infections in newborn babies.


What's Next in GBS Testing

In the future, hospitals may switch to a rapid test for GBS that can be done while the mother is in labor; tests that give immediate results are in the pipeline and already in use in some institutions. Doing the test at the time of labor might be a better strategy because a mother may be positive at 35 weeks but negative at 40 weeks when the baby is born or, more importantly, the opposite could be true. With a rapid test, mothers who are negative at the time of delivery wouldn’t be exposed to antibiotics, which are typically very safe, but can cause allergic reactions, and mothers who have become new carriers during this time frame wouldn’t be missed.

Another exciting development is a possible vaccine against GBS, which may be used in young teens to immunize them against GBS — similar to the way that the HPV vaccine (Gardasil) is currently used.

Continued efforts are being made to protect women and infants from the devastating effects of group B strep. There is very real hope that in the near future, GBS disease will be a thing of the past. For now, an easy test and treatment can help reduce the risks to your newborn.


Published on March 6, 2014. Updated on November 23, 2015.


Elaine Brown, MD, completed her residency in obstetrics and gynecology at Harvard Medical School. She has more than 15 years of experience in private practice.

Reviewed by Elisabeth Aron, MD, FACOG on October 5, 2015.