Two new studies released this week lend even more credence to the growing body of evidence that environmental factors may play a larger role in the development of autism than genetics.  

The first study, out of the University of California, San Francisco and Stanford University took a closer look at the high rate of autism noted among twins.  Previously, researchers believed that the high incidence of autism in twins suggested that genetics played a role.  But for this study, published in the Archives of General Psychiatry, researchers looked at 192 pairs of twins in California and used a mathematical model to determine that genetic factors increased the autism risk by about 38 percent. Environmental factors, on the other hand, raised the autism risk by 62 percent.

What was really interesting was the difference in autism rates between fraternal and identical twins.  You would think that if autism had a genetic cause, than if one identical twin were to have autism, the other would too, right?  But that's not necessarily the case.  In fact, only 60 to 70 percent of the identical twin cases had dual diagnoses.  A big number for sure, but not nearly as big as researchers would have thought if genetics played a larger role.  Of the 138 fraternal twin cases the researchers looked at, 20 to 30 percent had dual diagnoses - much higher than they would have anticipated for kids who share only half of their genes.

Another study released this week, and also published in the Archives of General Psychiatry, offers more evidence about the role of environmental factors in the development of autism.  For this study, researchers looked at the maternal use of antidepressants as it relates to the autism risk for kids.

They found that the risk of having a child with autism spectrum disorder was about twice as high among women who took antidepressants in the year before delivery. That risk was even four times higher when the women took antidepressants, such as Prozac, Zoloft, Paxil, and Celexa, during their first trimester. 

The researchers behind the Kaiser Permanente study warn that these numbers are just preliminary.  They don't want women to ditch their antidepressants altogether.  But it certainly raises some alarm bells and would be worth a consultation with your health care provider if you are pregnant or trying to get pregnant.

The opinions expressed by MNN Bloggers and those providing comments are theirs alone, and do not reflect the opinions of MNN.com. While we have reviewed their content to make sure it complies with our Terms and Conditions, MNN is not responsible for the accuracy of any of their information.