From birth through age 5, physical milestones are easy to monitor. Simply watch your child to see whether she performs the behavior.
At 1 month old, an infant is typically doing more than sleeping, eating and requesting a diaper change. She jerks her arms and waves her hands near her mouth. Position her prone for some "tummy time," and she'll turn her head to either side.
Jump ahead to 3 months, and tummy time is generally livelier. She should be lifting her head and chest from the bed and be using her arms to prop herself up. Her lower body should be kicking and stretching. A child's awareness of the world is growing at this age, so she should be able to bring her hand to her mouth and track movement with her eyes. And, finally, the much-anticipated parental payoff: a social smile in response to a familiar voice.
The next milestone is marked at 6 months, about the time her first tooth should erupt. At 7 months, she should be able to roll from stomach to back and from back to stomach. She may sit up alone, perhaps bracing herself with her hands. Fine motor skills also improve: She should be able to claw an object toward her, and when she's holding the item, she may trade hands.
By the end of the first year, she's likely Miss Independence, actively crawling and "cruising" (walking by holding onto furniture). She should be able to pull herself into a standing position and balance unaided, briefly. At mealtime, she may be using the pincer grasp (thumb and index finger) to feed herself small bits and, with assistance, drink from a cup.
Skipping ahead to 2 years, she should walk alone and run in an enthusiastic, if ungainly, manner. At 3, she should be climbing and descending stairs competently. By 4, she should be able to balance on one foot and dress herself.
During middle childhood (ages 6 to 8), the Tooth Fairy stops by as baby teeth begin to fall out and are replaced by permanent teeth. A growth spurt should hit her in later childhood (ages 9 to 11), though her male classmates likely won't start theirs until early adolescence (ages 12 to 14). Around this time is the arrival of menarche (her first menstrual period). Both males and females develop an increased need for sleep, although, because of school schedules and growing independence, it's very likely they won't get enough rest.
You may ask: What's happening internally as we see all these external changes? That's where we're heading next -- cognitive development.