162 Maternal Twins and Male Gender Bias in Autism Spectrum Disorders
discovered that while a higher maternal age was consistently associated with
higher risk for autism in children, the father’s age only had a negative impact
when the mother was younger than 30 and the father was older. The reason that
this is caused in older parents has yet to be solved, but genetic, environmental,
and hormonal effects may be potential contributing factors [1,2,56–59].
We suggest that the repeated, cumulative exposure to synthetic chemicals
that we are exposed to in the form of fragrances, herbicides and pollutants, in
our food, water, and air, most of which have teratogenic and/or mutagenic
effects over a long period of time, will increase the probability of parents to
pass on accumulated germ cell mutations to the fetus. We speculate that these
synthetic chemicals that cannot be broken down into innocuous chemicals
remain bound to certain proteins and stay dormant in the human liver, where
the majority of the toxic chemicals are detoxified. During pregnancy, a preg-
nant woman goes through enormous physiological and biochemical changes
and large amounts of hormones are released and, unfortunately, also unleash
the toxins that are present in the liver.
This phenomenon may partially explain the paradox of discordant maternal
twin pair twins, but also shed light on our hypothesis that even perhaps a few50
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<25 25–29 30–34 35–39 40+Rate (10,000 per population)Maternal ageFigure 6.6 Maternal age and autism in California (1990–1999). Advanced maternal age is
linked to a significantly elevated risk of having a child with autism, regardless of the father’s
age, according to an exhaustive study of all births in California during the 1990s by UC Davis
Health System researchers. The study found that advanced paternal age is associated with
elevated autism risk only when the father is older and the mother is under 30. Source:
Adapted from Refs [58, 59].