Pregnancy can be diagnosed as early as day 20-22 of pregnancy via ultrasound.  The fetal skeletons calcify at day 45 and can be seen on x-rays.  Pregnancy can be palpated in a dog with a normal weight sometime after day 26--though this can vary.  
Pregnancy lasts 63-66 days.  In the final stages mammary development will occur and if fed appropriately will gain weight.
Dogs can also experience false pregnancies that mimic pregnancy; dogs will even produce milk.
Your veterinarian can help you determine if your dog is pregnant.