Another idea is wet kitten food. Its packed full of carbs to help growing kittens but also helps promote weight in adult cats.
Has she seen a vet? The vet might also be able to provide you with suppliments to add to her meals to give her the added nutrients that she'll need. My vet gave me a box of powder that I had to add to my cats meals everyday when I first got her to prevent UTI. I'm sure they have something for preggo cats.