If you do have a scan then don't take it as 'gospel' when the amount of puppies are mentioned.

When Jo Jo had her 1st litter, immedietely after the first mating she slept the rest of the day (unusual for her), she became very clingy, her teats were enlarged, and we just sensed she was pregnant.

She did have a check up at the Vets at about 5 weeks into this and he could feel she was pregnant. We never got her scanned because very shortly after we could see her shape changing, at 6 weeks she started eating for england. We were told by our mentor (with 25 yrs experience) it was a waste of time having a scan, and if you are having it done to try to find out the quantity of puppies, in most cases it is not accurate. I also didn't choose to have a scan because i didn't want to stress her.
As Emily said The Book of the Bitch is really useful.

Good Luck with your girl.