Why do Doctors Test for Group B Strep in Pregnancy?

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  • Written By: Mary McMahon
  • Edited By: Kristen Osborne
  • Last Modified Date: 03 October 2019
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Doctors test for group B streptococcus bacteria during pregnancy to see if special precautions need to be taken during labor and delivery. There is a risk a laboring mother could pass the bacteria on to the baby, potentially causing severe health complications. By identifying carriers of group B strep early, doctors can intervene before the baby is infected by administering antibiotics during labor.

Many women naturally carry group B streptococcus bacteria as part of the normal vaginal and/or anal flora. These bacteria don't cause symptoms in many carriers unless they develop immune compromises and the bacteria have an opportunity to take over. In babies, who lack many immune protections, infection with group B strep can cause severe medical problems including meningitis, respiratory difficulties, and cardiovascular instability. Not all babies born to mothers who carry group b strep in pregnancy will become infected, but the risks make preventative treatment advisable for the safety of the baby.


The doctor checks for group B strep in pregnancy during the end of the third trimester, usually between 35 and 37 weeks. A swab will be taken from the vagina and another from the anus to see if any bacteria are present. If they are, no major changes need to be made to the labor and delivery plan, except for the addition of intravenous antibiotics during labor. The drugs will prevent infection. Patients with a previous history of reactions to antibiotics should make sure to discuss these so the doctor can prescribe an appropriate medication.

Women who have group B strep in pregnancy can also take antibiotics during the pregnancy, although the most critical period is during labor and delivery. Administering drugs to the baby after birth or when symptoms develop may be too late, and this is not recommended. Women who know they carry group B strep as a result of previous infections or positive test results can advise their obstetricians. The doctor will determine if another test for group B strep in pregnancy is necessary.

Having group B strep in pregnancy is not a sign of any wrongdoing on the mother's part. Although the bacteria colonize the genital area, this is not a sexually transmitted infection and many women carry streptococcus without being aware of it. Doctors recommend the test in the interests of making labor and delivery as safe as possible. This test reduces neonatal morbidity and mortality and is very simple to perform.


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