No, the chemicals in the body are different. You should see some signs by about 3 or 4 weeks, slightly fuller sides (less waistline), teats standing up and harder, bitch being a bit more cuddly. You can get a scan (ask your vet) but she either is or isn't, and if there is no veterinary reason why you need to know then I would just wait and see. Sometimes you can't tell right up until about week 8, if she is only having 1 or 2 and they are carried high up.
Hope she's not, there are more staffs in rescue than mongrels.