There is still much to be known about the causes of autism, but research has led to some conclusions.
There are some genetic risk factors that may lead to autism, such as changes in certain genes. If a parent carries one or more of these gene changes, then the child has an increased risk to develop autism. These genetic changes can also appear spontaneously in an early embryo or in the sperm or egg that fuse to create the embryo. The majority of these gene changes increase the risk for the development of autism spectrum disorder; most do not cause autism alone.
Autism spectrum disorder may also arise from environmental risk factors, including advanced parental age of either parent, pregnancy and birth complications, and pregnancies spaced less than one year apart. However, the increased risk associated with these environmental factors is very small. Environmental and genetic risk factors seem to affect early brain development. They also appear to alter how brain nerve cells, neurons, and regions of the brain communicate with each other.