Physical development refers to how your baby develops head control, strengthens neck muscles, learns hand coordination skills, crawls, and later learns to walk. Physical development occurs especially quickly during the first two years of your child’s life. Your baby’s birth weight generally doubles by six months and triples by his first birthday. You will see your baby grow between 10 and 12 inches in length (or height), and his body proportions will change during the first two years. When your baby was born, most of his body mass was in the head. As he gets older, the rest of his body catches up.