Feb. 8, 2010 -- The older a mother is when she gives birth, the higher her child's risk of autism, new data show.
A smaller effect also is seen for the age of the father, but only when the child is born to a father over age 40 and a mother under age 30.
The new findings come from a comparison of reported autism cases in California to state singleton birth records from 1990 to 1999. Over that time, there were about 5 million births and more than 12,000 autism cases.
- Women over age 40 are 77% more likely than women under age 25 to have a child with autism.
- Women over age 40 are 51% more likely than women aged 25-29 to have a child with autism.
- Women aged 35-39 are 31% more likely than women aged 25-29 to have a child with autism.
- Women aged 30-34 are 12% more likely than women aged 25-29 to have a child with autism.
- Women under age 25 are 14% less likely than women aged 25-29 to have a child with autism.
- Men over age 40 are twice as likely as men under age 25-29 to have a child with autism, but only if the mother is under age 25.
It's tempting to think that the trend for women to delay childbirth is behind the continuing rise of autism. But that's not the case. This trend accounts for less than 5% of the autism increase in California over the decade 1990-1999, calculate study researchers Janie F. Shelton, Daniel J. Tancredi, PhD, and Irva Hertz-Picciotto, PhD.
So what's going on? That isn't clear. Older parents' genes can undergo changes caused by aging and by the environment.
"We need to understand biologically why this is happening," Shelton tells WebMD. "It would be premature to tell older moms not to have a child. It could be the risk is associated with an exposure, and avoiding the exposure would be more important than not having kids at age 40."
Exactly what is a woman's risk of having a child with autism? The figures from the study come from 1990 to 1999. Autism cases increased throughout that decade -- and throughout the next decade, too.
"By now the rates are much higher," Hertz-Picciotto tells WebMD via email. She also notes that the cases counted in the study are only those receiving services from the state of California, so the risks she and her colleagues calculate are underestimates.
From 1990 to 1999, the risk of having a child with autism was:
- 1.6 per 1,000 women under age 25
- 2.3 per 1,000 women aged 25-29
- 3.1 per 1,000 women aged 30-34
- 3.85 per 1,000 women aged 35-39
- 4.4 per 1,000 women aged 40 and older
It's clear that a parent's age is only part of the autism puzzle, notes Geraldine Dawson, PhD, chief science officer for Autism Speaks, a group that advocates for autism research.
"This study does not say advanced mother or father age causes autism," Dawson tells WebMD. "This is one risk factor among many factors that contribute. In the majority of cases, we are not going to find that any one factor accounts for any individual child's autism. Parental age is just one risk factor that is interacting with other genetic and environmental factors that lead to a child developing autism."
The Shelton paper was in the Feb. 8 online advance issue of the journal Autism Research.