Mismatching is common.
Conventional wisdom holds that a baby's height, weight, and head circumference should be in the same percentile. While that's often the case, don't fret if your baby's percentiles seem out of whack. "Both scenarios are common," says Cynthia Ferrell, M.D., an assistant professor of pediatrics at Oregon Health & Science University, in Portland. During a growth spurt, a baby's length may shoot up, or he may gain weight quickly, causing his growth-chart percentiles to diverge.
Big heads happen.
Of all your child's organs, her brain grows most rapidly, with the head expanding right along with it. That's why your pediatrician so conscientiously measures head circumference -- to ensure that the brain is developing properly. One way to tell that a baby is significantly undernourished is that she stops gaining height and weight, while her head continues to grow at a normal rate (the body automatically reserves calories for all-important brain growth). If a baby's head seems particularly large in relation to her height and weight, the doctor may measure the parents' heads, since this characteristic runs in families.
Preemies are speedy.
Although babies born preterm (before the 37th week of pregnancy) are small at birth, they grow faster than full-term newborns -- most likely because they're completing what would have been their in-utero growth. By 2, most premature babies have caught up with their full-term counterparts. But there are exceptions. Being a multiple or being extremely premature, for example, can affect a baby's future size, says John Walburn, M.D., a professor of pediatrics at the University of Nebraska Medical Center, in Omaha. "Some of the smallest preemies never quite catch up, no matter how well nourished they are."
The genes have it.
Is your nine-pound newborn destined to be a linebacker? Birth size can predict adult size, but only when it reflects hereditary factors. "Many large babies drop to lower percentiles after six months of age. We say they're 'finding' their genetic potential," says Dr. Walburn. In other words, if you and your spouse are short and slight, your child is likely to be too.