CHICAGO (AP)- Traci Fries never dreamed that by helping to keep her baby safe from crib death she was contributing to a separate and growing medical problem.

From the nape of his neck to the top of his skull, the back of 10-month-old A.J. Fries' head is perfectly flat.

The culprit, doctors told the Chicago mother, was putting A.J. to sleep on his back - the increasingly common practice credited with a steep decline in crib deaths nationwide.

Fries, 26, says she noticed the problem not long after A.J.'s birth last September and asked about it at every checkup.

"The doctor kept saying, 'Oh, as he grows older it should fill back out.' It never did."

She has since been advised to see a specialist to see if her son needs to wear a special helmet to mold his head back into shape.

Skull flattening can occur because newborns' skulls are soft and malleable, which allows the growing brain to expand. The medical name is positional plagiocephaly, a condition some pediatricians had never heard of before the nationwide "Back to Sleep" campaign that started in 1994 to reduce the risk of Sudden Infant Death Syndrome, or SIDS.

Despite concerns over the rise in plagiocephaly, SIDS is still the leading killer of infants in their first year. Some doctors worry that parents' concern about flat heads may lead to a return to stomach sleeping - something experts stress should still be avoided.

"If I have to trade a decrease in SIDS with getting asymmetry in the back of the head, that's a great trade," said Dr. Frank Vicari, a pediatric plastic surgeon at Children's Memorial Hospital in Chicago.

Thousands of children are affected, says Dr. Louis Argenta, chairman of the plastic surgery department at Wake Forest University and director of the North Carolina Center for Cleft and Craniofacial Deformities.

Lea Menchaca's plagiocephaly was noticed at the age of three months, when her father was bathing her.

"He said, 'Her head's crooked,' and I said it was no big deal," said Lea's mother, Traci Menchaca, 29, of Houston.

Menchaca had a cousin whose baby was a stomach sleeper and died of SIDS, so she was compulsive about making sure Lea, now 2, and her older brother slept on their backs. But she "completely freaked out" when a pediatrician referred her to a neurosurgeon and a pediatric plastic surgeon.

The specialists identified the problem and recommended a DOC Band, an open-topped helmet approved in 1998 for treating plagiocephaly.

Usually harmless and easily treated, plagiocephaly has led at worst to needless tests, unnecessary operations and big expenses, specialists say.

But some plagiocephalic infants must wear plastic helmets for several months, and in extreme cases surgery may be needed to correct such things as misaligned ears, which may be prone to infections, Argenta said.

The American Academy of Pediatrics recommended periodically repositioning infants' heads while they sleep and allowing "tummy time" when they're awake to help prevent flat spots.

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