underweight baby?

So I'm 33 weeks pregnant and I've only gained 9 pounds since the beginning. My doctor has sent me to a specialist to check if my baby girl is at the right measurements and gaining weight. I'm not dieting in any way, I eat healthy but I also still enjoy Taco Bell and ice cream. I don't know how to gain weight so my womb nugget will gain weight. Anyone else in their 3rd trimester with possible IUGR? Or experienced moms with IUGR and had a healthy chunky baby? I need to put my worries away