The investigators on the current study looked at 565 children with autism born in metropolitan Atlanta between 1986 and 1993, and compared them to a set of children without autism, as well as to children with other developmental disabilities, such as mental retardation, cerebral palsy, hearing loss or vision problems.
Overall, low birth weight was associated with a twofold increased risk for autism, but the risk was higher for girls than for boys.
For all low-birth-weight children, the risk for autism accompanied by other developmental problems, such as mental retardation, was higher than the risk of developing autism alone.
There was also double the risk for developing autism in babies born prematurely, although this was primarily due to a more than fivefold increased risk in girls born early.
"This was one of the first studies that had a large enough sample to look at girls," Molloy said. "They really were able to tease out what is different about boys and girls."
Even so, the elevated risk for autism seen in low-birth-weight and preterm babies was much lower than that linked with cerebral palsy, mental retardation, hearing loss or vision impairment.
"It's not yet clear why being small or being born too soon could lead to these problems but, Schendel said, "[these factors] could be a marker for an impaired fetus, one that has a neurological problem which is retarding its growth. On the other hand, being small or being born too soon may be related to factors that could harm the neurological development of the fetus such as infection during pregnancy."
The findings support the idea that there are different kinds of autism and different mechanisms underlying those cases, Molloy said.
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