Usually between 35-37 weeks of pregnancy. This is done via a simple swab of the vagina and rectum.
Streptococcus agalactiae, otherwise known as Group B Strep (GBS), is a bacteria that approximately 20-25 percent of pregnant women have in their vagina and rectum. This bacteria can be passed to a baby as he or she passes through the birth canal at the time of delivery. GBS infections in newborns can lead to infections in the lungs, bloodstream, and nervous system, and can have severe consequences. To decrease the numbers of newborns who could have these complications, all pregnant women are now screened for GBS to see if they are positive. Those women who are positive are then treated with antibiotics in labor.
Screening usually happens between 35-37 weeks gestation. Additionally, if a woman has GBS found in her urine culture (which is done at the beginning of her prenatal care and any other time a concern for a urinary tract infection arises), this swab will be skipped, and it will be assumed she is still a carrier. Women who go into preterm labor or whose bags of water break early will also be screened before the 35-37 week mark. Lastly, women who are having a scheduled C-section need not be treated during their delivery but should still be screened in case they do in fact deliver vaginally.