GBS is a bacteria which can grow in the vagina it is normal vagina flora but it can make babies very sick (septic) temperatures etc and needing antibiotics.

It is a flora which can come and go which is why they test later in pregnnacy so if you are positive they will give you antiboitics when either in established labour of if your membranes rupture.

If you are positive and come in to hospital very close to birthing and membreanes are intact and go right before baby comes this clears out all flora from vagina, so you may not get antis if this happens.

Some people choose not to have the antibiotics are just have babies temperature monitered before every feed for 48 hours.