NEW YORK - Like any expectant mother, Kai Walter, six months pregnant, has lots to get done before the big day. One of her most important errands: an upcoming trip to the West Coast, where she has an appointment to take off her clothes and be photographed. Not for some magazine cover, a la Demi Moore, but for her own personal collection of pregnancy memories. The idea is to artistically capture her blossoming belly in all its glory, something more and more women are doing these days. Or they might be making a plaster "belly cast" of their changing form....