Can you be underweight & a surrogate mother?

I am really short 4 feet 9 inches and have always been really skin no matter what or how much i eat, my blood test always came out good thou so my doctor never worried much about my weight, I weight 80 pounds and according to my height should be 86 pounds. However when i got pregnant i was 80 pounds and i got pregnant at the 1st try. Not i really want to be a surrogate mother and I have a couple interested and was wondering if i am healthy would this be an issue? can the doctor deny to do the procedure? even if the intended parents where fine with it? I am doing all i can to gain weight but i am curious