Autism occurs in some families more frequently, therefore genetic factors or mechanisms are likely to be one of the major causes. This is supported by the fact that the prevalence of autism in twins who grow up together is much higher in identical twins compared to non-identical twins. Gene environment interactions might also play a role which means that environmental factors might alter gene regulation (epigenetics).
The vulnerable window, the time period in which humans are particularly vulnerable to developmental damage and to adverse health effects of environmental stress, is during pregnancy and early childhood. Various associated risk factors are under discussion, these include the use of certain medications such as paracetamol, low birth weight, advanced maternal age, vitamin deficiency (especially vitamin D and folic acid in the first few months of pregnancy), stress and infections during pregnancy or infancy. The hypothesis that infant vaccines with mercury-containing serum might play a role has been refuted. However, this hypothesis led in general to more focus on environmental chemicals.
In particular, chemicals that affect the child's developing brain, as well as those that affect the endocrine or immune system, may be relevant. These include heavy metals such as lead and mercury, pesticides such as organophosphates, flame retardants, polychlorinated biphenyls and phthalates and some volatile organic compounds (VOCs) such as trichlorethylene and styrene. Studies investigating correlations between exposure to these chemicals and autism often do not lead to consistent results because not all exposure factors are considered or even known. In addition, the specific vulnerable window (e. g. month of pregnancy) has to be taken into account. This also applies for example to air pollution caused by road traffic which might play a role in the first few months of life.
In a large-scale EU project called MiND (http://mind-project.eu/) the researchers are investigating the genetic causes of autism and the gene-environment interaction. It is to be hoped that this research will provide concrete indications to reduce risks for pregnant women and for people who wish to have children.
