Group B streptococcus — also called group B strep — is a common bacterium often carried in the intestines or lower genital tract. Although group B strep is usually harmless in adults, it can cause complications during pregnancy and serious illness in newborns. If you're pregnant, your health care provider will likely recommend a group B strep test during the third trimester.
During a group B strep test, your health care provider will swab your vagina and rectum and send the samples to a lab for testing. In some cases, you might be given instructions on how to collect the samples yourself. Because you can test positive at certain times and negative at other times, you'll need to repeat the group B strep test each time you're pregnant.
If the group B strep test is negative, no action is needed. If the group B strep test is positive, you'll be given antibiotics during labor to prevent group B strep disease in your baby.