GBS is transient, it goes in about a 6 week cycle. So they test you at 36 or 37 weeks, because I guess they figure that's how they'll catch the most cases that would actually be positive at the time of delivery. If you tested much earlier, the bacteria may have come or gone by the time of delivery. And if you tested later, you'd miss a good number of the deliveries.
From what I've read, you are at highest risk for transmitting GBS to your baby if you've had a bladder infection with GBS, if you labour for a long time with waters broken, if you have a fever during labour, and if you deliver before 37 weeks. If you go into labour before 37 weeks, they might just hook you up to the antibiotics anyways, just to be "safe". I understand that the IV antibiotics have cut the number of deaths from GBS in half. However, the risk of transmitting GBS to your baby is quite small, over all. And antibiotics come with risks as well. So you should do some reading and decide for yourself what you'd like to do about the swab, and treatments, I'd say.

Here's a thread I posted a while ago asking about this, and Kelly and Alan gave some great info.

https://www.bellybelly.com.au/forums...rs-others.html

All the best!