Jan 7, 20117:28 PM ET
By Rachael Rettner, MyHealthNewsDaily Staff Writer
"People are going to manifest the disorder in different ways, and that could be because there are different sets of genes, [or] different sets of environmental factors," that contribute to how the disorder presents itself, said Alycia Halladay, director of research for environmental sciences for Autism Speaks, an autism advocacy organization that funds autism research.
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Babies that have been exposed to certain pharmaceuticals in the womb, including valproic acid and thalidomide, have been found to have a higher risk of autism.
Thalidomide is a drug that was first used in the 1950s to treat morning sickness, anxiety and insomnia. The drug was withdrawn from the market after it was linked with birth defects, but is currently prescribed for a severe skin disorder and as a treatment for cancer.
Valproic acid is a medication prescribed for seizures, mood disorders and bipolar disorder, according to the NIH.
4. Parental age
As parents grow older, they have a higher risk of having children with autism, Halladay said.
A study published last February found that women who are 40 years old have a 50 percent greater risk of having a child with autism than women who are between 20 and 29 years old.
Researchers aren't sure why parental age may influence autism risk, but it might be related to genetic mutations that occur in the sperm or the egg as parents grow older, Halladay said.
5. The development of the brain
Particular areas of the brain, including the cerebral cortex and the cerebellum, have been implicated in autism, Mao said. These brain areas are thought to be responsible for concentration, movement and mood regulation.
Irregularities in the levels of neurotransmitters, such as dopamine and serotonin, have also been tied to autism, Mao said. Problems regulating dopamine can lead to problems with concentration and movement disabilities, while troubles controlling serotonin levels can result in mood problems.
While scientists cannot say definitively what causes autism, they have come a long way in the last decade, Halladay said.
For example, researchers originally thought there might be just a single gene or a few environmental factors that are linked to autism, but now evidence has shown there are likely to be more.
"I think our knowledge has increased, and the way that we go about looking for potential genes and environmental candidates has improved," Halladay said.
"We're thinking about a new model — it's not just going to be just one gene or one environmental factor, it's more complex than that," she said.