Ensure Milk

Ob wants me to start drinking Ensure milk because I haven't gained any weight I'm very petite, my normal weight is 95lb. In the beginning of my pregnancy I lost so much weight from morning sickness that lasted all the way up to I was 4 months pregnant I'm back at my normal weight and stuck at it I'm now 21 weeks and can't gain anymore, do you think I should start drinking them? Do you think it'll help? I've literally tried everything to gain weight before I got pregnant and nothing works