This fixation on size begins moments after your baby's born. Everyone wants to know his height and weight stats. From then on, the pediatrician will measure him at every checkup. While inches and pounds are important, growth is key. First developed 30 years ago, government growth charts gauge how his measurements compare with those of other kids of the same age and gender. "We don't worry as long as a child grows steadily, but an unusual jump in height or a sudden falloff in growth can be a sign of a health problem," says Elizabeth Littlejohn, MD, a pediatric endocrinologist at the University of Chicago Comer Children's Hospital.
During his first year, your baby grows up to 10 inches in length. However, his early size isn't very predictive of his final height or weight. He'll add about five inches during his second year, and then starting at age 3, he'll grow about two and a half inches per year until a final growth spurt kicks in during puberty. Girls reach their adult height around 15 (the average American woman is 5'4"), while boys don't top out until 16 or 17 (the average American male is 5'9"). A school-age child who falls below the 5th percentile for height is considered to be of "short stature," a clinical term meaning he's shorter than 95 percent of kids his same age and gender, whereas a child who's in the 95th percentile is taller than all but a few of her peers. Average height is anywhere between the 40th and 60th percentiles. You can look at your family tree to figure out your child's future potential: Height is about 80 percent genetic. A child whose parents are both 6' is likely to be the tallest kid in kindergarten. If both parents are unusually short, their children probably will be short too. But even kids who have average-height or tall parents may inexplicably stop growing -- a condition known as idiopathic short stature (ISS). These children are significantly shorter than 99 percent of their peers, and will remain small as adults. "Boys generally won't grow to be more than 5'4", while the girls might hit 4'11"," says Dr. Littlejohn. Late bloomers, on the other hand, are small for their age but still growing at a normal rate and will eventually have a growth spurt and catch up to their peers.