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As scientists try to determine which environmental factors may lead to autism, they are paying closer attention to what happens to fetuses in utero, researchers say.

Such factors play a larger role than genetics in causing the disorder, according to a new study from researchers at the University of California, San Francisco, and Stanford.

“Autism had been thought to be the most heritable of all neurodevelopmental disorders, with a few small twin studies suggesting a 90 percent link,” said Neil Risch, a UCSF geneticist who is the director of the UCSF Institute for Human Genetics and one of the co-authors of the paper in a statement. “It turns out the genetic component still plays an important role, but in our study, it was overshadowed by the environmental factors shared by twins.”

The fact that the disorder has such an early onset suggests that the causes occur early in life, possibly during the mother’s pregnancy, the researchers say. But what in the environment may be causing autism is still a mystery, with research just beginning to explore that question, according to Dr. Lisa Croen, director of the Autism Research Program at Kaiser, who was another co-author on the study.

“It is really everything that you can think of that’s not genetic, including things that people are exposed to at their home, their workplace, out and about in the world,” Croen said in an interview. Environmental factors could include toxic exposures to chemicals, infection or other diseases, medications for health conditions, as well as lifestyle factors such as diet and sun exposure, she said.

“What we think is that it’s really a combination of genetic factors and environmental factors that are combining at a point of critical development of the baby,” Croen said. “A lot of this we think is happening during gestation when the fetus is in the womb and developing.”

Historically, autism was thought to stem from bad parenting, with women branded “refrigerator mothers” for being so cold that they failed to bond with their children and caused the child to go into a state of withdrawal. That theory has been wholly debunked, and scientists are now urging more research into how environmental factors affect genetic factors. 

“It’s really looking at biological factors that are not coded in the DNA, but that could influence how the DNA is expressed,” Croen said.

One environmental factor under study is the effect of anti-depressants on the developing fetal brain.

Another study, also published on Monday in the Archives of General Psychiatry and co-authored by Croen, found that exposure to Selective Serotonin Reuptake Inhibitors, a type of anti-depressants, in early pregnancy may increase the risk of autism.

Mothers of children subsequently diagnosed with autism were twice as likely to have at least one anti-depressant prescription in the year prior to delivery, and over three times as likely to have a prescription in the first trimester of pregnancy.

Yet, the researchers cautioned that this is the first study that has reported this association, and more research is needed, especially to confirm if the medication is causing the problem or the mother’s underlying condition, such as depression or anxiety, which led her to take the medication.

Serotonin, the neurotransmitter often thought to play a role in depression, may also be a factor in autism. “There is a growing body of evidence in the scientific literature pointing to serotonin abnormality in autism,” Croen said.

Yet, she cautioned: “It’s really important that people don’t stop taking their meds if theyr’e on them. This study by no means suggests any change in clinical practice.”

Two major studies currently under way should shed more light on the environmental factors contributing to the disorder. One is the Early Autism Risk Longitudinal Investigation, which according to its website "will follow 1,200 mothers of children with autism at the start of another pregnancy and document the newborn child's development through three years of age." Kaiser is one of four sites nationally where the study is being conducted, and researchers are currently seeking subjects who live within two hours of the San Francisco Bay Area, with research being conducted at locations in San Jose and Oakland. 

The other study, sponsored by the Centers for Disease Control and Prevention, called the Study to Explore Early Development, is the largest study in the United States to help identify factors that may put children at risk for autism. Kaiser in Alameda and Contra Costa counties is one of six centers nationally where the study is taking place.