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.