Pregnancy and Birth Environment may Affect Development of Autism in Twins

A recent study of twins with autism spectrum disorder suggests that genetics alone cannot be responsible for the high rates of autism found in fraternal twins.

A recent study of twins with autism spectrum disorder (ASD) suggests that genetics alone cannot be responsible for the high rates of autism found in fraternal twins. Environmental influences, such as parental age, low birth weight, multiple births, and maternal infections during pregnancy also can greatly increase risk for ASD in twins.

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

The researchers focused on 192 pairs of twins, both identical and fraternal; at least one of the twins in each pair suffered from autism. Identical twins share 100% of their genes, and as such, studying them can help researchers determine the degree to which a disorder is inherited or genetic. Comparing the results to the fraternal twins, who share about 50% of their DNA, allowed researchers to understand how environmental influences affect the risk of ASD.

The autism science and advocacy organization, Autism Speaks, aided in the research by providing material from their Autism Genetic Resource Exchange (AGRE).

The study, known as “The California Autism Twins Study,” suggested that both genetic and environmental factors significantly increase risk for ASD in fraternal twins.

Researchers discovered that an estimated 38% risk of autism is associated with genetic heritability, while 58% is associated with the environment that all twins share while in the womb, and possibly even early infancy.

These results held true for both males and females.

It was not clear, however, whether specific time periods of the pregnancy such as early pregnancy, late pregnancy, or birth were associated with the higher risk of ASD; further, researchers could not determine specific risk factors—such as parental age, maternal nutrition, maternal infections during pregnancy, and premature and/or underweight birth—that contributed to the increased risk, said Lajonchere.

"Indeed, multiple-birth pregnancies are themselves associated with increased risk of developmental disorders such as cerebral palsy and autism. This speaks to the importance of further study on what prenatal and perinatal factors increase risk beyond that of inherited genes," Lajonchere concluded.

The study was published online today in the Archives of General Psychiatry.