The only time GBS status is carried over from one pregnancy to another is if the baby is infected with GBS after delivery. If a baby has GBS sepsis, then all future pregnancies are automatically treated as GBS+.
If the mom is GBS+ but the baby has no after effects, then that GBS "episode" is null and void during the next pregnancy and testing is done again.
This is not always the case. I have had many friends whose OBs said it wasn't necessary to test in subsequent pregnancies since they were + in a previous one :(
Obviously, any testing done at anytime is really only a snapshot of the NOW at that moment though and should not be considered relevant beyond 3-5 weeks since your culture/status can change during that short of time, yet we also have OBs and CNMWs who are testing in early pregnancy and taking those results, too.
IMO, the routine testing isn't necessary nor is the ABX treatment. Just another intervention.