When will my baby sit up, crawl and walk? What is considered "normal" when it comes to reaching milestones? What I tell new parents about their baby's development is this: Infants' growth once was viewed as a sort of developmental elevator. Baby went from floor to floor, stopping at each level to learn a new skill. But there's more to that story. Babies build their skills based on the environment they find on each floor. If the interaction with you or other caregivers is responsive and enriching, she gets back on the elevator with even more skills, and the ride to the next stage is much smoother. Knowing how your baby acquires new cognitive and motor skills not only teaches you about him, it's also a worthwhile reminder that every infant is a little different (and it may even help you avoid the neighborhood race to see whose baby sits or crawls or walks first). Here are the average time frames and descriptions of some of the most important first-year accomplishments.

BIRTH TO 3 MONTHS

Lifts and turns head Stretches out arms and legs Opens hands and swipes at things Holds eye contact and studies faces Smiles Makes first sounds

During the first year, motor development simply means that baby is learning to lift more of his body off the ground. When you place him on his stomach soon after birth, he gradually inches his head off the floor -- from barely clearing the surface during the first month to raising it an inch or two the next. By 3 months of age, most babies can lift their heads higher than their bottoms.

His limbs soon learn new tricks. In the first month, a baby's arms mostly lie curled up around his body, but by the end of the third month, he stretches out both his arms and legs and begins to move them around more freely.

Then baby's hands start to explore. Newborns tend to keep their tiny fists tightly closed, but during their second month they begin to unfold them partially and swipe aimlessly. By the third month, the hands remain half-open most of the time. It's at this stage that babies realize their hands are wonderful toys and -- most important -- a part of themselves. Your baby will play with his hands in front of his face, exploring one hand with the other and sucking on his fingers and fists.

His sight is improving as well. Early on, infants see best when objects are 8 to 15 inches away. For the first month, their eyes are crossed and can hold a fixed gaze only briefly. But sometime during the second month, your baby makes genuine eye contact and studies you, at least for a few seconds. Once he can see you clearly, you'll soon be charmed by his smiles as he begins to hold your gaze. Around 2 or 3 months, coos, squeals and gurgles expand to include more drawn-out vowel sounds such as "aaah," "eee" and "oooh." Sounds become louder, and screeching and laughing start.

At the end of 3 months, you've survived the fitting-in period. Baby's fussiness while adjusting to life outside the womb has subsided (somewhat!), and he knows to whom he belongs. Because his needs have been consistently responded to, he has developed trust -- the most powerful stimulator of infant development and the foundation of your child's self-esteem. Most parents now feel more comfortable with the two R's of parenting: reading their baby's cues and responding in a way that works.