Don't be surprised if your sleeping baby sometimes kicks her legs and punches her arms. She's not having a nightmare. "Babies have a very immature nervous system, and it's hard for them to control their reflexes and responses to environmental stimuli such as noise and temperature," says Kenneth Wible, M.D., a pediatrician with Children's Mercy Hospitals and Clinics, in Kansas City, Missouri. This means that those erratic movements are involuntary and usually harmless. To be sure, take your fingers and lightly hold down the arm or leg that's twitching. If the movement stops, no problem. But if it doesn't, if it occurs while your baby is awake, or if she has whole-body spasms, you should seek further evaluation, including a screening for epilepsy. Twitching that's not caused by a neurological disorder usually decreases between ages 2 and 4 months.