Nearly 15 years ago, the American Academy of Pediatrics (AAP) first recommended that parents put their babies to sleep on their back.

That simple piece of advice cut the death rate from sudden infant death syndrome (SIDS) by more than half. An unexpected result has happened, however: flattened heads.

The flattening — a result of babies’ spending so much time on their back — most often happens on the back of the infant’s head and is usually more noticeable on 1 side. This flattening may broaden the head and face. In severe cases, the flattening may push forward 1 side of the face, creating an uneven appearance.

No one’s sure how common flat heads are. But around half of infants have some amount of flattening of the head by 2 months of age. Only about 1 in 5 of those have severe changes, according to a 2013 articles in the journal Pediatrics.

To avoid a flat head — medically known as deformational plagiocephaly or positional plagiocephaly — try these tips:

• Parents should still place babies on their back for sleep.
• When babies are awake, put them on their tummy for a while. This eases pressure on the back of the head and helps babies build shoulder and neck strength.
• Alternate which direction you place your child in the crib each night. Your child will then alternate which direction he or she looks out of the crib.
• Don’t hang objects above your child’s head. Put them on both sides of the crib so he or she will turn his or her head to look at them. 
• Dress your child in clothes that allow for freedom of movement. 
• Alternate sides if you bottle-feed your child. 
• Reduce the use of car seats when not traveling in the car, as well as other types of seats like bouncers in which babies are positioned on their backs.
• Pick up your child often. The more time your child is held in your arms, the less time he or she is lying down with pressure to the head.
• If your child develops a flat spot on his or her head, see your health care provider.