A mother cat does not start producing actual "milk" until one to two days after the kittens are born. During the pregnancy (at the end) she produces "colostrum" which is made up of antibodies and other nutrients that her newborns need. (It is very, very important for the kittens to get the colostrum- it helps build their little immune systems and protect them from illness and disease). For the first day or two of the kittens' lives, they will get colostrum. After the colostrum is gone, they'll get milk.