This happened to my Phoebe when she was pregnant. She went off her food at 3 weeks pregnant and for the whole of her pregnancy I really struggled to get her to have anything at all, and to top it all if I'd have known how many pups she was carrying (11) I would have been even more worried than I already was. She was on a raw diet and I didn't want to give it up so when I saw what was happening I just gave her a selection of all the things she usually had and watched what she was eating. I increased anything that she was still happy to eat and decreased the rest. I had to stop using herbs because she wouldn't eat things with the herbs on, then it was certain meats she wouldn't eat, then it was the cereal breakfast, then it was the rest of the stuff. I did give her a bit of cooked meat because she was willing to take that and I used more high value stuff which I would normally reserve for the last couple of weeks (think most peeps use puppy food at that point), but things like eggs and offal which meant she was having higher nutrients but very small portions.
Hope all goes well, but I really do know what you are going through.