According to The California Autism Twins Study, environmental influences including parental age, low birth weight, multiple births and maternal infections during pregnancy may significantly increase the risk for autism spectrum disorder (ASD).

Researchers explained that an estimated 38 percent of risk is associated with genetic heritability and 58 percentage with the environment that twins share during pregnancy and early infancy.

It has been well-established that genetic factors contribute to risk for autism, Clara Lajonchere, Ph.D. and vice president of clinical programs for Autism Speaks, said.

We now have strong evidence that, on top of genetic heritability, a shared prenatal environment may have a greater than previously realized role in the development of autism, she added.

The researchers studied 192 pairs of twins, both identical and non-identical. At least one of the twins in the pair had autism.

Study of identical twins, sharing 100 percentage of their genes, helped researchers determine the degree to which a disorder is inherited or genetic and comparison to fraternal twins, sharing around 50 percentage of their DNA, showed how environmental influences add to the risk of ASD.

The results showed that since the prenatal environment and early postnatal environment are shared between twins, probably some of the environmental factors affecting susceptibility to autism exert their effects during these critical periods of life.

However, the study failed to pinpoint the specific time period (early pregnancy, late pregnancy or birth) or the specific risk factors (parental age, maternal nutrition, maternal infections during pregnancy, premature and/or underweight birth, etc.) that contribute to the increased risks.

Published online in the Archives of General Psychiatry.