I find they are more cuddly and loving, wanting to be with you all the time, and the first physical signs are usually the nipples turning pink and going a bit hard and standing up. This is usually at about two to three weeks. Other than that, no signs until they either suddenly get very round or actually go into labour if they have only a small litter or just one puppy carried high up.
another indication of pregnancy can be a white discharge from or around the 35th day.............sometimes before . sometimes they are sick, go off food, and sick up bile. Also blue rings round the teets at around 3 weeks.
And some dogs have no signs at all and deliver a perfectly healthy litter of puppies.