Physical Development

At 4 1/2 months, your baby probably understands when you say her name, although she might not respond to it yet. She can also identify her mother's voice, even in a crowd, and will turn her head when she hears it. If your child doesn't seem responsive to your voice, or doesn't start babbling within the next month or so, there could be a problem with her hearing.

Babies with auditory problems may still be startled by loud noises, but will have difficulty imitating speech. If this seems like an issue with your child, ask your pediatrician to do a hearing test. And before you panic, remember that there are many causes of hearing loss, most of them correctable. For instance, babies who are prone to ear infections may have fluid build up in their inner ears, which can interfere with hearing.

Social Development

Your baby is a bundle of energy at this stage, waking at the crack of dawn to practice his new skills, ranging from quietly playing with the activity center in his crib to shrieking so exuberantly that it wakes the neighbors. Some active babies wake up several times a night, while others wiggle, roll over, and twist their bodies in their sleep, propelling themselves all over the crib.

Help your child release some of her extra energy during the day with frequent exercise sessions--try teaching her to roll, if she doesn't already. Allow her short play periods at night to help her get back to sleep. Never restrain an active baby with a leash or harness--this will dampen her growing desire to learn new skills, and could endanger her physically.

Intellectual Development

One of the many new talents your baby is perfecting is the ability to taste every object, large or small. This requires her to grasp items with one or two hands, maneuver them from one place (generally the floor) to another (her mouth), and hold them in place while she sucks or chews on them. Although it's hard not to panic when you see her happily mouthing your old running shoe, try not to show your disapproval too vehemently. Instead, applaud her exploratory skills while replacing objectionable objects with toys designed for chewing. Be sure to wash chew-toys frequently in very hot water, and keep them away from other children to avoid the spread of germs.

Note: The information above offers general guidelines, but all babies develop differently, and few hit their milestones precisely when the conventional wisdom says they should. If your child was born prematurely, you may want to use your due date as a baseline for following baby's development.