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Pregnancy & Prenatal Testing

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Third Trimester: Group B Streptococcus Screening

Group B streptococcus (GBS) are bacteria that are present as part of the normal flora in the vagina and gastrointestinal areas of about 25% of women. This is different than the more commonly known Group A streptococcus that causes strep throat.

GBS is usually not a problem, except when it is present in the vagina during delivery. In this case, it can spread to infect the uterus, amniotic fluid, urinary tract, and any incision made during a cesarean section. At delivery, when the baby passes through the birth canal, the bacteria can be inhaled or ingested.

Infected infants may display symptoms within 6 hours of birth or as late as 2 months of age. If untreated, the baby may become septic, develop pneumonia, suffer hearing or vision loss, or develop varying degrees of physical and learning disabilities.

To best assess the risk of a pregnant woman infecting her baby at delivery, the U.S. Centers for Disease Control and Prevention (CDC) recommend that all pregnant women be screened for GBS between 35 and 37 weeks of gestation. Specimens from the mother's vaginal and rectal areas are collected, and within 24 to 48 hours the laboratory can determine if Group B strep bacteria are present. If the bacteria are present, or if the woman goes into labor before testing can be completed, it is recommended that she receive antibiotics intravenously during labor.

GBS may be present in a woman's vagina off and on throughout her pregnancy. Therefore, testing for GBS earlier in her pregnancy is not useful in determining whether or not it will be present when she goes into labor and might risk spreading it to her baby during delivery. Only testing late in the pregnancy, between 35 and 37 weeks, is useful. Likewise, treatment with oral antibiotics before labor has not proven to be effective in preventing GBS infections in the newborn.

March of Dimes: Group B strep infection
CDC: Group B Strep (GBS)