NEW YORK -- The newborn's journey is often tortuous and about one in 10 -- and more than half of twin babies -- come into the world with a flat spot on their head, according to a new study.

Such unusual shapes are nothing to be concerned about, experts say, and tend to disappear as the child grows. However, it is possible that such babies may be at greater risk for a flattening of the head that comes with too much time spent on the back, according to Dr.

Wiebke K. Peitsch and colleagues from Harvard Medical School in Boston.

The "back-to-sleep" position is a lifesaver, cutting the risk of Sudden Infant Death Syndrome (SIDS). But since 1992, doctors have seen more infants with a flattening of the skull on the back of the head, usually due to a combination of too much time on the back -- be it in crib, swing or car seat.

"Pediatricians are seeing an increase in the number of posterior flattening since the American Academy of Pediatrics began encouraging parents to put babies on their backs. It has not been established whether babies are born with flat spots, or if it starts after birth," study co-author Dr. Constance H.

Keefer, assistant professor of pediatrics at Harvard Medical School, told Reuters Health.

The Harvard team wanted to see if they could identify newborns at risk of developing head flattening, known as plagiocephaly.

Plagiocephaly can often be corrected by varying the child's crib position and giving the child supervised "tummy time" when awake. It is sometimes treated with a molding helmet, the researchers note.

According to a report in the December issue of the journal Pediatrics, the researchers examined more than 200 healthy newborns 24 to 72 hours after delivery. Premature babies or those requiring prolonged care were excluded from the study.

They found 13% of infants had at least some degree of head flattening and 11% had other unusual head shapes.

In twins, 56% had head flattening. Overall, the risk of having a flat spot was higher if a suction or forceps was used to help delivery, if there was a prolonged labor, an unusual birth position, if the baby was a boy or if it was the mom's first baby.

Flattening on the right side of the head occurred more frequently than on the left, probably as a result of passing through the birth canal. The researchers believe that babies may find it more comfortable to sleep with their head turned toward the flat portion, possibly making the problem worse.

"The message in general to the Academy, pediatricians, and parents should be that putting babies on their backs to sleep is important, but that allowing babies to lie on their stomachs is also important for development. If an infant has a flat spot at birth, these babies should definitely be helped to lie on their side and parents should change their child's position in the crib," said Keefer.

Jim Kudo, director of the Division of Hospital and Surgical Services for the American Academy of Pediatrics (AAP), told Reuters Health that the recommendations to prevent SIDS are up for review in April, but he doesn't think the AAP considers head flattening to be a major concern.

The incidence of SIDS has declined since 1992 when the AAP began recommending that infants be placed on their backs to sleep.

Before 1992, when infants were placed on their stomachs to sleep, any head flattening in babies tended to be on the front of the skull, according to the report. While there has been a dramatic drop in cases of flattening on the front of the head since the "back to sleep" recommendations were issued, flattening on the back of the head has become more common, the researchers note.

They suggest that since flat areas on the head are easy to diagnose, parents should make an extra effort to prevent further head flattening. They can make sure that their child has supervised tummy time when awake, and limit the amount of time the baby spends in a car seat, the researchers say.