Researchers at the University of Utah School of Medicine showed women who give birth at age 35 or older are 1.7 times more likely to have a child with an autism spectrum disorder, compared with women between the ages of 20-34.
First author Dr. Deborah A. Bilder said children diagnosed with autism spectrum disorder also were nearly 1.8 times more likely to be the firstborn child.
"The results of this study give us an opportunity to look more closely at these risk factors for children across the autism spectrum, and not only those diagnosed with autism," Bilder said in a statement. "This shows that further investigation of the influence of prenatal factors is warranted.
The study, published online in the journal Pediatrics, said a possible explanation for the correlation of firstborn children might be that parents are reluctant to have a second child if the first is diagnosed with autism spectrum disorder.