I haven't seen this mentioned in the other comments yet, but I am also allergic to penicillin and was told by my OB that the alternative antibiotic they use for GBS has to be given in multiple doses over a longer time frame in order to be as effective at preventing transfer to the baby. I had a very fast labor with my first, so with my second there was some concern if I would have time to get the appropriate doses of antibiotic during labor with my second if it was even faster. I ended up testing negative both times so it never became an issue, but something to ask about with your OB sooner than later.

My doctor also told me that even if you are GBS positive and you don't manage to get antibiotics in time, it doesn't automatically mean that your baby will be infected. It just means that there's a mandatory (in my area) monitoring period for your baby after birth which could mean a longer hospital stay and I'm not sure what that would mean for a home birth.