If you're pregnant, the Centers for Disease Control and Prevention recommends a group B strep screening between weeks 35 and 37 of pregnancy. Your doctor will take swab samples from your vagina and rectum and send them to a lab for testing. A positive test indicates that you carry group B strep. It doesn't mean that you're ill or that your baby will be affected. It simply means the potential for newborn infection exists, and you can take steps to protect your baby.
If you've already given birth and your doctor suspects your baby has group B strep disease, a sample of your baby's blood or spinal fluid will be sent to a lab for evaluation.
Group B strep infection and group B strep disease are diagnosed when the bacteria are grown from cultures of the fluid samples. The cultures take several days to grow, so it may be two to three days before you receive the results of the lab analysis.