Even though you shouldn't use a coverlet (blanket) or comforter in your baby's crib, a blanket or comforter can come in handy as a play mat or exercise pad for tummy time under your watchful supervision. Tummy time--the time your baby spends on the floor during the day doing "push-ups"--helps your baby develop upper body coordination and posture and neck strength, prerequisites for crawling and other physical skills.