GBS or group B strep is a pathogen that approximately 1/3 of all women carry. It is not a sexually transmitted infection and it doesn't make women or their partners sick. However, newborns born to women with positive GBS vaginal/rectal cultures are prone to sepis (wide spread infection) or illness because of their immature immune systems. Only about 5% of babies born to a GBS+ woman would actually become sick, but the illness can be so rapid and the newborn could potentially die, therefore, we perform universal testing and treatment for all women. Testing occurs usually between 35-37 weeks gestation unless preterm labor or premature rupture of the amniotic sac occurs. In the latter case we would test and treat immediately. Otherwise, we routinely swab (with a q tip type swab) in the vagina and slightly into the rectum during the last month of pregnancy. We do the rectal swab because the majority of strep actually lives in the intestinal tract and that area is still a risk factor for transmission to the baby. If the culture is positive we give intravenous antibiotics at the onset of labor and continue to give them intermittantly every 4 hours until delivery to prevent infection in the newborn.
Some patients may be positive with one pregnancy and negative with another. The reason is because strep can colonize (grow) and once it has grown to a certain level it is picked up on culture and said to be capable of causing infection in the baby. Once we treat you, it may stay at low enough levels that it is not a problem in future pregnancies. However, we will often treat anyway (especially if you'd prefer) if you have been positive with other pregnancies and negative with the current one just in case the bacteria grows in the time from doing the test to the time of labor even though current guidelines do not say this is necessary. We also repeat the test if it is negative and you don't deliver within 4 weeks to see if it turns positive in that time.
For those of you going natural and concerned about the IV with being strep positive, we recommend treating it with antibiotics and then we can hep cap your IV between doses so you can still feel free to get up and move or walk. You can even soak in the bath once you are treated (and preferably if your water bag is intact).