BOSTON -- Doctors at 15 autism treatment centers nationwide have collected data that show an association between autism and children conceived through in vitro fertilization, WCVB-TV in Boston reported.

More than 1,400 families of autistic children completed medical questionnaires that asked, among other things, if their child was conceived with assisted reproductive technology, or ART. IVF is the most common type.

Twelve percent of those questioned said yes, a number Dr. Patricia Davis said is 10 times the most recent national average published by the Centers for Disease Control and Prevention.

Johanna Nelson participated in the survey. Two of her four children have autism. Her oldest son, Daniel, 7, is severely impacted. He is non-verbal, attends a special school and receives a long list of special therapies to help him in his day-to-day life.

"It's a big, lingering question in our minds every day, as to why he was chosen to be autistic," said Nelson.

Nelson said both of her autistic sons were conceived via artificial insemination.

After Daniel was diagnosed at 22 months old, she said, "That was one of the first questions that I asked, 'Could it be that the fertility drugs gave him autism?'"

Davis said her work began as an effort to confirm what she and her colleagues were noticing anecdotally.

"A lot of moms that we were interviewing told us that they had had their children using assisted reproductive technology," said Davis. "It was quite hard to determine if this was something we were noticing or if it was a true relationship."

The findings, she said, "confirmed what we had suspected."

What's more, the data collected showed that autistic children conceived with ART tended to have more severe cases.

"Which is interesting, because the increase in numbers that we've seen over the last few decades has been the less severe form of autism," said Davis.

However, Davis cautions, "I don't want to leave anyone with the impression that we think IVF causes autism. That's not the case. It just leads to so many more questions."

"For me this is too early," said Alice Domar, a fertility specialist and executive director for the Domar Center For Mind/Body Health.

Like Davis, she said there are many other possible explanations. The age of both parents can raise the risk of autism. Domar said multiple births, so common among families who seek help getting pregnant, can also boost the risk.

"We're seeing a pocket here, but that does not mean that infertility treatment causes or contributes to autism," said Domar.

"If you're taking somebody who may not have perfect eggs or perfect sperm to start with, you're going to have a greater risk of a lot of different problems. People really want to know, is something they're doing putting their children at risk? I'm not that worried right now."

It has been 32 years since the first "test tube" baby was born. Research continues to show that overall, these children are healthy.