In the early 1990s, parents were told to lay their babies on their backs to prevent sudden infant death syndrome, or SIDS. While the cause of SIDS is unknown, back-sleeping is believed to reduce the risk.

This relatively new practice has surely saved lives … but has also been linked to one consequence — an increase in the number of babies who develop flat spots on their heads. Plagiocephaly (play-jee-ah-SEF-uh-lee) or flat-head syndrome, occurs when an infant’s head becomes misshapen. This can happen when a baby’s head is resting on one side for extended periods of time, such as where the back of the skull presses against the mattress.

According to a recent study, roughly one in five babies under 6 months old develops a flat spot due to lying on his or her back while sleeping.

A common remedy for the problem is an expensive, custom-made helmet. The helmet can cost anywhere from $1,500 to $3,000. Babies wear the helmet for 23 hours a day for two to six months.

The study suggests that helmets produce no more improvements in the baby’s skull shape than if the parents had done nothing at all. The study tracked 42 babies ages 5 to 6 months with flat spots. The babies wore the custom-designed helmets for 23 hours a day for six months. Researchers then monitored 42 additional babies with similar flat spots who didn’t wear helmets.

After two years, researchers could find no significant difference between the babies.

If you are a parent concerned about a flat spot, be sure to check with your child’s pediatrician to rule out craniosynostosis (crane-eo-sin-os-TOW-sis), or bones fusing together prematurely, which could require surgery.

Want to avoid flat spots? Put baby on her tummy. Babies who get more tummy time and spend less time in car seats face better odds of never having a flat skull.