Rolling over used to be a 4-month milestone, but now it is happening later, at around 5 months. Put your baby on his stomach for at least a half-hour a day to build upper-body strength. Now is also a good time to begin playing simple sound-gesture games, such as "Where is Thumbkin?" and "Itsy Bitsy Spider." Your baby may begin to make trilling, growling and lip-smacking sounds.
Sitting up unassisted usually occurs around 6 months, but it's fine to prop your baby up for short times before this. "It lets the baby see his world and use his hands to explore. It also strengthens his core muscles," says Ari Brown, M.D., an Austin, Texas, pediatrician. Babies 4 to 6 months old can be propped up between 45 and 90 degrees for 15 minutes or so at a time.
For more information about your baby's milestones, check out these Baby Steps.