When your dog enters the sixth week of her approximately eight-week pregnancy, her condition starts becoming obvious. Before that, most expectant dogs gain relatively little weight, and that's healthy. That's the point at the which the fetuses start rapidly growing and gaining, so it's reflected in the maternal body. That's also the time when your dog requires extra nutrition to feed those expanding puppies.
Canine pregnancy lasts approximately 63 days, although a few days earlier or later falls within the norm. In early pregnancy, your dog might gain a little weight, but not a significant amount. By the sixth week, as her abdomen gets larger, you'll also notice her nipples darkening and becoming more prominent. In her last two weeks of pregnancy, as her girth increases, you'll notice the puppies moving around inside her. At that stage, it's safe for your veterinarian to X-ray her abdomen so you'll know how many puppies to expect.