Putting babies to sleep on their backs is preventing sudden infant death syndrome (SIDS), but too much time on their backs it might also be leading to an increase in flat spots on babies' heads.
As reported by NBC News (Nearly half of babies have flat spots, study finds - July 8, 2013), a recent study found that 46.6% of babies had some form of plagiocephaly ("oblique head").
The solution includes varying the side of the head that is placed down when the baby goes to sleep, and also increased "tummy time." Tummy time, even when infants are still very young, is so important. Babies come out flexed up into a ball, and as their muscles begin to relax and their bodies straighten out, tummy time helps the muscles in their neck become active. Babies can typically begin to lift their heads and clear their mouth/nose as early as 10 days after they're born! Within the 1st month, they should begin to get a little bit of clearance, within the 2nd month they should be able to lift their whole head off the ground, and by the 3rd month your baby should be able to hold their heads up and support themselves on their elbows. Once your baby can support their upper body on their elbows, they're typically within ~6 months of crawling!
Download Tummy Time Tools from the APTA's website for quick tips on how to position, carry, hold, and play with your baby to promote muscle development in the child's neck and shoulders and avoid the development of flat areas on the back of the baby's head.