Judging pregnancy can be tricky. A female can have a false pregnancy and exhibit all the signs of pregnancy. Females can also be pregnant and then if not fed the proper diet or if they are exposed to certain chemicals they can absorb the pups. It is very important to get the bred mom on a proper diet and do not expose her to chemicals, medicines or excessive heat.
This is an interesting article that every breeder should read:
Birth Defects in Dogs.