If you can find a dog food that has 18% or less protein, you would be better off. Higher fat is what you are looking for, when feeding pregnant/nursing moms. I use Wellness Senior, but not everyone can($12 for 8lbs). Nutro is a good food too, get the Senior.

Sometimes I put peas and lentils, sometimes I don't. My mix(about 50lbs every 3-4 months) varies every time I make it. I like adding Kashi GoLean, Kashi Crunch, any of the fun granola cereals, whole in shell nuts(walnuts, pecans, filberts, etc), raisin bran, brown rice, oatmeal, whole flaxseed(2-3lbs each time I make the mix, it works wonders on their coats and skin), tons of tri-colored and wheat pasta, wild rice if you can find it, rice chex, wheat chex, puffed wheat and rice, etc.

I would stick with a lower protein, higher fat content mix. High protein is harder on their kidneys and makes them more susceptible to allergies(itching and scabbing ickiness). If a female has problems holding weight regardless of what you are feeding her, I wouldn't breed her again. It's just not worth it, IMO, to have the babies undersized and her body wasting away. Takes alot of time to get them back to weight. I have one female, Reggie, who had 14 babies and lost a dramatic amount of weight. The babies were all good-sized though. We haven't bred her again as it just takes too much out of her and she was exhausted all of the time.

Hope that helps a bit! And no, Lentils and dried peas aren't poisonous If they were, my rats would have been dead years ago, LOL.