Recognition has grown over the
past decade that aspects of our environment may also contribute to autism.
However, despite substantial research, no one environmental factor has yet been
found to be a definite cause of autism.
The most widely used research technique to examine environmental
risk factors for autism is epidemiology, which examines how often, and why,
diseases occur in different groups of people.
Several environmental factors during prenatal life have been
linked with autism. Bacterial or viral infections
in the mother during pregnancy have been found to slightly increase the risk of
autism in the offspring. This could be due to the passage of harmful infectious
organisms from the mother to the fetus through the placenta, or because the
immune response of the mother may be detrimental to the developing brain of the
fetus.
Other factors in the mother that may be related to offspring
autism include a folic acid deficiency at the
time of conception, the presence of gestational diabetes and the
use of certain antidepressants during
pregnancy, but no conclusive evidence exists for any of these links.
Being an older parent, particularly an older father, is also
thought to increase the risk of having a child with autism. As males get older,
the number of sperm that contain de novo genetic mutations
increases.
Some of the de novo genetic mutations will have
minimal or no effect on the resulting baby, but some mutations can lead to the
brain developing differently.
Several studies have
found that fathers who are over 50 at the time of conception have a greater
chance of passing on de novo mutations and also a greater risk of having a
child with autism.
An obvious, but very important, observation is that not all
people who are exposed to these factors are diagnosed with autism. One possible
explanation for this is a phenomenon called gene-environment interaction, which
is when the genetic make-up of two different people leads them to respond
differently to an environmental factor.
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