Her vulva will appear swollen much like it does when the dog is in heat. Usually, her vulva will return to "normal” size once the heat cycle has concluded.
Her nipples will become much more pronounced, which is similar to the changes in other mammals, as the mammary glands begin to prepare for the nursing period.
She may seek more attention, which is a behavioral result of the change in hormonal balance and physical comfort. Again, a woman might be able to communicate cramping or discomfort but your pit bull will simply seek comfort from you.
After 3 weeks or so, it is usually possible to see a pronounced belly. Obviously, this is because the puppies in the future litter are growing; and you may be able to see or feel them moving around too. It is important that you start preparing your home for the birthing of the puppies. Make a safe whelping area where she can rest as the due date nears and get her used to it two weeks ahead of time. She might lose her appetite over the last two days and she might begin to shiver or present signs of nesting too.