Feed satin balls. http://www.amysacres.com/docskingshe...satinballs.pdf
It helps with weight gain on skinny dogs, but your dog could use the extra nutrition.
I understand the want to not abort the puppies... I'm not keen on that idea either. However, you are going to have to make sure she is extra strong and a bit chunky for either the birthing of those pups or the c-section. The pregnancy part is not the scary part, it's the whelping part that is scary. Hopefully you took good care of her throughout her life and she is strong.
I've seen pregnant mamas pick and not want to eat. I don't want to say its normal, but its not unheard of either. Try the satin balls.. It should stimulate her appetite and give her calories that count.
Try not changing her food. You should make one more change, and that should be to puppy food. Its higher in fat and helps give you nice, plump puppies.
Keep in mind, everything you give to her, goes directly to her puppies..
I have also heard that slightly warmed cottage cheese *spoonful* will help stimulate her appetite.
Do you know how many puppies she's going to have? Due to her age, you need to be there for her when she whelps. She might get too tired and stop, which is not good...