Pregnant or Nursing Females:
The big needs of pregnant and nursing females are adequate levels of protein and fat. These can be supplied using kitten or puppy chow, a good brand (such as Science Diet, Iams, Eukanuba, or any of a number of other good brands). Also table scraps can be useful, being palatable, as long as you avoid heavily spiced or salted or sugared items which are hard on the rat's system. (Note: avoid high fat foods.)
If a doe (female) appears to be expecting a good-sized to huge litter (based on the size of her abdomen), I will take away the lab blocks entirely in favor of the richer foods so that she has the best possible chance of providing adequate nutrition for the growing rat kittens inside of her. I will continue with this (with fruits and veggies, too, and sometimes a little grain mix or whole wheat bread) after the babies are born, but then I will add goat's milk for the extra protein, calcium, fat, and palatability. This is good for putting weight on a thin mother or helping boost up the runts. Sometimes a large litter will nurse the flesh off of a mother--she just doesn't eat enough to make up for what they are taking from her body by nursing. This is, in my opinion, VERY BAD for the female and should not be allowed. If necessary take the babies away for several hours a day and let her rest and eat--feed her lots of tasty food, rich food, eggs scrambled in butter, oatmeal with whole milk or goat's milk, canned kitten or puppy food, whatever she'll eat that's nutritionally rich. Meanwhile offer the babies the same foods separately. Hopefully this will let her catch up and quit losing condition.