While the theory of stress to the mom during fetal brain development is certainly plausible, as a woman with 5 kids (who all have neurological issues in some form including one with HFA) living in a subtropical zone of the US, I have suffered from allergies to molds (high humidity here) for as long as I can remember.  

These allergies have always been worse just prior to an intense storm and during the high humidity that follows.  During this time I feel like my brain is inflamed!  It is much more difficult to think clearly & process info as I normally would.

I have read that the fetus picks up abnormal factors from mom's immune system & can't help believing that this is a significant factor.  I have to wonder if all the women in this study lived in areas that are susceptible to tropical weather?  If not, then a woman who was pregnant while living in a dry area like AZ, for example, would not likely suffer from the constant stress to the immune system endured by a woman in a highly allergenic environment.