I have a 8 month pregnant mare ?

I have a 8 month pregnant horse. She is a lil ribby and her topline has dropped. How can i get weight on her? She is eating 4 scoops of XTN a day 2 and 2 and on a round bale 24/7. i just found out she was pregnant a month ago, before that she got 1 scoop 1/2 and 1/2. Im giving her supplements. She was pregnant before and lost the baby and had been with my stallion. well i moved and had her checked again and the vet said she wasnt anymore so she had lost it and got pregnant again. But the vet couldnt tell it was 3 days to early. So if anyone can help please do! I dont want her to be skinny when she drops it. It's her first. She wasnt skinny before.and not really skinny now but just thin