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 occurred, however: flattened heads.

The flattening—a result of babies' spending so much time on their back—most often occurs on the back of the infant's head and is usually more pronounced on one side. This flattening may broaden the head and face. In severe cases, the flattening may push forward one side of the face, creating an asymmetrical appearance.

No one's sure how common flat heads are. Statistics vary a great deal, from one in five cases for a mild form to one in 500 to 600 cases, the AAP says. But doctors have seen a significant increase in flat heads in the past decade, says a 2003 article in the journal Pediatrics.

To avoid a flat head—the medical term is 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.
• Relieve pressure on the back of the head when you lay an infant down for sleep by propping the child slightly to one side. Alter sides nightly.
• 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 overuse car seats when the child is not in a car. When in a car, move the car seat often from one side to the other.
• 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 the head, see your doctor.