I find that my bitches will eat the puppy complete for a while, then go off it completely a few weeks prior to, and just after whelping. They are then put onto an anything I can get into them diet ;-) This includes raw mince, tinned creamed rice, chicken, tuna, in fact anything just to get them to eat :-p I have even resorted to tinned food mixed into their puppy complete, whch worked quite well for one bitch, but only for a week
It's a bit like pregnant women. We have cravings, and dislikes (I hated coffee with my last one

) and so do dogs. Unfortunately the only way they can tell us is by refusing the food. That's where the hair goes grey(human, not dog :-D ) ad the bank balance goes into the red :-)