First, take a deep breath and try not to stress. If you’ve tested positive for group B strep, you're now armed with knowledge that can protect baby.
When you go into labor, you’ll be put on an antibiotic drip (usually penicillin, unless you're allergic) that flows down into your amniotic fluid, blood and birth canal to wipe out some of the bacteria that could be potentially harmful to baby. With the help of the antibiotic, baby should be just fine. GBS-positive women who don't receive the antibiotic, though, are 20 times more likely to pass the bacteria on to their baby.
Guidelines say you should begin receiving the antibiotics four hours prior to delivery, so make sure your hospital is aware of your condition and in possession of the vaccine before your due date. You should also make an effort to get to the hospital in plenty of time to be put on the drip, and don’t be shy to let the nurses know that you need your vaccine when you arrive.
Expert: American College of Obstetrics and Gynecologists. Your pregnancy and birth. 4th ed. Washington, DC: ACOG; 2005.