This article is from the Group B Strep FAQ, by email@example.com (Cheryl Sandberg) with numerous contributions by others.
Typically, babies are exposed to GBS during labor and delivery; they may
also be exposed after the mother's membranes rupture ("water breaks").
Babies can come in contact with GBS if the bacteria travels upward from the
mothers' vagina into the uterus; they may also be exposed to it while
passing through the birth canal. The babies become infected when they
swallow or inhale the bacteria. There is also evidence that GBS may cross
intact membranes to expose the baby while it is still in the womb. There it
may cause preterm births, stillbirths or miscarriages. However, these may
be caused by a variety of factors; other infections, stress, genetic
defects for example- so be sure that any of these complications are
investigated fully even if you are colonized with GBS.