One of my sons is extremely underweight and has been his entire life. Well, he had so little body fat that the doctors were worried that he would not hit his major growth spurt in his early teen years. The docs said that if the body thinks there is not enough reserve that it will just skip growth spurts.
Well, he has hit a major growth spurt. He has gone from 5'2" and 66 lbs. (12.1 BMI) to 5'5" and 84 lbs. (14 BMI) in a pretty short time frame.
He is still underweight, but he is gaining weight. Finally! He'll be 15 in January. I am so glad he isn't skipping his growth spurt.