I had it with my first. They just ran antibiotics as soon as I checked in on a 4 hour rotation.
I had it with my first pregnancy and didn't even know. When I went into labor, they gave me antibiotics, and once that ran through my system, I could focus more on actually getting my son out. It is nothing to worry about and is very common. They can definitely treat it in the hospital when you go into labor.
There's nothing to worry abt Group B Steep is normal in adults from what I hear its just harmful to babies so they give the antibiotics during labor so baby will be safe. As far as size if your Dr doesn't have any concerns then its probably nothing to be too concerned about. Try not to stress. Good luck mom!
If your doctor says not to worry, then don't worry. Antibiotics are all that's needed. You doctor can only guess at the size of the baby and can be off by quite a bit.