Nearly 30 years ago, the American Academy of Pediatrics first recommended that parents put their babies to sleep on their back. That simple piece of advice cut the death rate from SIDS (sudden infant death syndrome) 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 one side. This flattening may broaden the head and face. In severe cases, the flattening may push forward one side of the face. This creates 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 article in the journal Pediatrics.

To prevent 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 and under supervision, put them on their tummy. This eases pressure on the back of the head and helps babies build shoulder and neck strength. Place your baby in tummy time while awake and supervised for short periods of time. Begin this soon after hospital discharge. Gradually increase tummy time to at least 15 to 30 minutes total each day by age 7 weeks.
• Interact with your baby during "tummy time" and provide objects for your baby to see and touch.
• Alternate which direction you place your child in the crib each night. Your child will then alternate which direction they look out of the crib.
• Don't hang objects above your child's head. Put objects on both sides of the crib so they will turn their head to look at them.
• Dress your child in clothes that allow for freedom of movement.
• Alternate sides if you bottle-feed your child. Don't let your baby fall asleep while bottle-feeding. Milk or formula pooled in your baby's mouth can cause tooth decay.
• 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 they are lying down with pressure to the head.
• If your child develops a flat spot on their head, see your doctor.