In response to recent concerns and publicity about an increase of “flat heads” in infants who sleep on their backs, First Candle/SIDS Alliance provides the following information for your use in addressing this issue.
  
Also known as “positional plagiocephaly”, experts agree that it is rarely a serious condition and can be avoided if parents make a conscious effort to change the direction the infant’s head faces when they are on their backs.  In the vast majority of the cases when a baby develops a flattened head, the problem usually resolves with time (by 6 months to one year of age) as the baby spends more time awake and begins to roll to different positions by themselves.  The use of helmets (usually costing in the range of $2,000) or surgery to correct this problemis rarely needed.

• Alternate the direction you place the baby to sleep in the crib;
• Move the mobile, mirror or other object of interest to the opposite side of the sleep and/or play area;
• Switch the arm with which you hold the baby, especially during feeding times;
• Place the car seat on opposite sides of the car;
• Provide the baby with lots of supervised “tummy time” during the day when they’re awake.

This will not only enhance the shape of the head, but will strengthen the upper body muscles that are not used as much when babies sleep on their backs.  It also allows for quality “exercise” or “play” time between you and your baby!
First Candle/SIDS Alliance recommends that parents consult with their physician if they are concerned about a more abnormal flattening of the head to eliminate the possibility of a more serious condition such as torticollis (a tightening of the muscles of one side of the neck) or craniosynostosis (a very rare premature closing of the sutures of the skull).

Information taken from www.firstcandle.org