Couples who undergo a specific form of fertility treatment are at slightly elevated risk of producing babies with autism or intellectual disabilities, scientists found in the largest analysis of its kind.

"We wanted to know where the risk is and around which [in vitro] treatments," said Dr. Avi Reichenberg, the study's lead investigator at the Icahn School of Medicine of Mount Sinai Medical Center in Manhattan.

He and his collaborators at King's College in Britain studied 2.5 million births of children in Sweden, where records are fastidiously maintained on fertility, births and outcomes.

There were 103 babies diagnosed later with autism and 180 who were found to have a form of mental retardation; both risks were about 1 percent.

A type of in vitro fertilization technology involving the extraction of sperm from infertile men, known as intracytoplasmic sperm injection, or ICSI, slightly increased the risk of producing offspring with autism or intellectual disabilities. No other form of in vitro fertilization carried such risks, Reichenberg said.

Reichenberg emphasized, however, that infertile couples hoping to produce a family through any form of reproductive technology should not stop their efforts because of his findings.

Alycia Halladay, senior director for environmental and clinical sciences with Autism Speaks, a national advocacy organization, said the study, on the whole, provides a comforting message to parents.

"The only reason for ICSI would be after vasectomy," she said. "With this particular procedure, they found a fourfold risk in autism."

Autism Speaks helped fund the investigation.

Reichenberg said the research was designed to produce a working hypothesis and to direct medical scientists into a further examination of the issue.

Complete results of the investigation are published in Wednesday's Journal of the American Medical Association.

"There is something that is very important to remember," Reichenberg, a professor of psychiatry and preventive medicine, said Tuesday. "Autism and mental retardation are rare outcomes.

"This [research] tells clinicians who are treating couples with fertility problems about a possible risk and that this is something they need to communicate [to patients]. But what this means is that most of the children who will be born [as a result of IVF technology] will be perfectly normal."

Numerous in vitro fertilization technologies have been developed over the past four decades, resulting in 5 million babies born worldwide. The first so-called test tube baby was born in 1978 in Britain.

Doctors not connected with the research said concerns had risen in recent years about older parents, the use of IVF technology and the increasing number of children diagnosed with autism-related disorders. Scientists wanted to know whether there was a correlation.

Dr. Avner Hershlag, chief of the Center For Human Reproduction at North Shore University Hospital in Manhasset, said the study is important for doctors practicing reproductive medicine.

"No work has ever been published to such an extent," Hershlag said Tuesday.

"The significance is that this encompasses a large number of infants in a Swedish registry who were monitored carefully and in a systematic way."

He said the study also revealed that IVF babies born as twins or other multiples are at greater risk of intellectual disability, which is why the practice of transferring more than one fertilized egg is discouraged.

His center alone, he said, has produced more than 25,000 healthy IVF babies over the years.

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