ATLANTA (AP) -- Katie Clapp knew something was wrong with her newborn boy, but it took two years and dozens of doctor visits before he was diagnosed with the most common inherited cause of mental retardation.

By that time, Clapp had given birth to another child with Fragile X Syndrome. If she had known about the genetic flaw, she said, she wouldn't have had more children.

A survey released Thursday by the Centers for Disease Control and Prevention is the first to look at the often long diagnosis for the incurable disorder that affects about 50,000 people nationwide.

Don Bailey, who led the survey from the University of North Carolina, said the findings raised the question of whether all babies should be screened for Fragile X.

Newborns are routinely screened for defects and disabilities that would benefit from early treatment, but doctors aren't sure how much can be done for Fragile X because it is difficult to diagnose. The testing of all newborns would be expensive.

Bailey said parents would see the most benefit from earlier screenings.

The survey of 140 families with a child with Fragile X found that 40 percent of the women had given birth again before learning they were carrying the gene mutation that causes the disorder.

The mutation occurs on the X chromosome and disrupts production of a protein important for certain brain functions, including reasoning. About one in 4,000 boys is born with Fragile X, and as many as one in 269 girls is thought to carry the mutation.

Individuals can carry the mutation without having the syndrome, and some may be only mildly affected.

Diagnosis can be difficult since children with Fragile X have no distinct physical traits to clue doctors to the syndrome. In the study, one in four parents went to doctors more than 10 times before learning the problem was Fragile X.

Further complicating the diagnosis is the fact that not all cases of mental retardation are inherited.

Clapp said that when she noticed her son Andrew was disabled, doctors assured her there was no reason not to have more children. Andrew was 2 1/2 and his sister Laura was still an infant when Clapp learned she was carrying Fragile X. Laura now has the syndrome, too.

"It's a really hard thing to sentence somebody to," said Clapp, who lives in Massachusetts and works with the Fraxa Research Foundation, an advocacy group for parents of Fragile X children.

Clapp said the expense of testing all babies for Fragile X would be worth it to spare parents the anguish of not knowing what's wrong.

"It's hard to describe how much easier it became when we found out we weren't just bad parents," she said.

Copyright 2002 The Associated Press. All rights reserved.