Feeding more early in pregnancy tends to add unnecessary fat, which will make delivery more difficult and increase the risk of complications. Be prepared for a 3 to 10 day period of a lack of appetite in your cat around the third week of pregnancy. This is a common occurrence and is not that much of a concern if the cat is at least eating a little bit. If she completely stops eating for more than 1 to 2 days, consult your veterinarian.
During the last 4 to 6 weeks of pregnancy, the kittens begin to grow rapidly. This puts a big demand on nutrients and the mother-to-be will need more energy. During this final part of pregnancy, your cat’s weight should gradually increase by 40 to 50 percent. For this reason, during the last 4 to 6 weeks of pregnancy, your cat should be gradually fed more and more food until she is eating about 50 percent more food by the time she delivers. This means that if your cat normally eats one cup of food a day, you should slowly increase her food until she is eating 1½ cups of food a day. Also, it is a good idea to gradually switch her food over to a diet that contains more calories per mouthful. This includes growth/kitten food or a nursing/lactation diet. By feeding your adult cat kitten food, she will be able to consume the necessary nutrients to provide for her rapidly growing brood. A lactation diet is also a good choice. During the last 1 to 2 weeks of pregnancy, you may want to let your cat eat as much as she wants. During the last week or so, energy demands are great and your cat may need to eat up to twice her normal diet.
from http://www.petplace.com/cats/feeding-the-pregnant-cat/page1.aspx