"Physical development consists of both gross motor (GM) and fine motor (FM) development," says Cheryl Wu, M.D., of LaGuardia Place Pediatrics in New York City. "While every child develops at his own pace, there are certain milestones I expect most of my patients (90 to 95 percent) to achieve by their third birthday." By three years of age, a child can balance briefly on one foot, go up and down the stairs with ease and alternating feet, walk in a straight line, and ride a tricycle well. He can draw a circle, and perhaps even a picture of Mom. Three-year-olds may push buttons, zip up their own jackets, and insert items (like keys) into holes.

"A good doctor will assess a child's development by asking questions, observing, and testing a child directly," says Dr. Wu. Do your best to make their physical activity fun -- and as much of a family affair -- as possible, says Brenda Rogers, M.D., a general pediatrician at Children's Mercy Hospitals and Clinics in Kansas City, MO. "Encourage your child to ride his tricycle while you're out jogging or riding your own bike, or just run around the park together."