Updated on February 27, 2025.
Autism spectrum disorder (ASD) is a developmental condition affecting about 1 in 36 children in the United States, according to the Centers of Disease Control and Prevention. ASD starts in early childhood, typically by age 3, and is generally considered to be a lifelong disorder.
The signs and symptoms of ASD vary from child to child and typically exist in combination. They can change over the years and often affect every aspect of daily life. A child with autism may have speech and communication issues. They may display repeat behaviors, have specific and limited interests, and follow strict routines. Many kids with ASD face physical health and development challenges.
Scientists believe there isn’t a single cause of autism. Instead, many factors are linked to increased risk, says Victoria Chen, MD, a pediatrician in New Hyde Park, New York. “The simplest way to talk about risks for autism is that there are genetic factors and environmental factors. Some combination of the two is causative for autism, but we don’t know enough to be able to say what that combination is,” Dr. Chen says.
As more research is conducted, some risk factors for autism are becoming clearer, while others are being ruled out.
Risk factors for ASD
Studies suggest children with autism share some common genetic and environmental risk factors. Having them doesn’t mean a child will develop ASD, but that their odds may be greater than someone without the risk factors.
Genetics and ASD
Evidence strongly suggests there’s a genetic component to autism. Having a family history of ASD increases a child’s risk, and more than 800 genes have been associated with the condition.
For example, studies of identical twins—who have the same set of genes—suggest a 60 to 90 percent risk that if one identical twin has ASD, the other will also develop ASD. Twin studies show only a 20 percent risk in nonidentical (fraternal) twins, who share half of the same genes.
Siblings also have a higher risk for ASD. If a family has one child with autism, there’s an increased risk of having another child with the condition.
Other gene-related risk factors for ASD include:
- Being born with another genetic disease, such as Down syndrome, fragile X syndrome, or Rett syndrome
- Being born male; compared to girls, boys are about four times more likely to have a diagnosis
- Being born to older parents, though more research is needed to confirm this link
Some cases of autism are tied to spontaneous mutations, as well. These are gene changes that aren’t inherited from parents and have no known cause.
Environmental risk factors and ASD
Pregnancy and infancy are critical periods of development for growing brains. During these times, fetuses and babies are highly sensitive to environmental exposures. These factors are not related to genes and are not thought to cause autism alone. But they may nudge genes in ways that increase the chances for developing ASD.
Environmental risk factors that may raise ASD risk include:
- Exposure to air pollution or pesticides before or after birth
- Being born to a mother with diabetes or obesity, or who develops a fever or has a disorder of the immune system during pregnancy
- Being born early (premature) or with a low birth weight
- Having difficulty during labor and birth that decreases oxygen to a baby’s brain
- Exposure to toxic metals, such as mercury, lead, or arsenic, during pregnancy or after birth
- Lacking certain vitamins and minerals (prenatal vitamins) during pregnancy
It is important to know there may be other risk factors and that none of the risk factors listed here are a known cause of ASD. Researchers continue to collect data on these and other possible risks.
What's not a risk factor?
Ever since a small, discredited 1998 study suggested a link between autism and childhood vaccines, there have been questions about the connection. Dr. Chen says very good epidemiological studies have examined it extensively. “There’s nothing there,” she explains. No link is known to exist between the two.
Furthermore, she adds, “The changes that occur in the brain that are associated with autism happen really, really early, in utero and when children are born, long before they receive vaccines.”
Children younger than 9 months may show symptoms like lack of eye contact, inability to express emotions like joy and surprise, and no interest in responding to their name. “That’s how early the changes are happening,” Chen says.
Supporting children with ASD
Diagnosis and interventions are critical to helping children with autism. The American Academy of Pediatrics recommends that healthcare providers look for signs of developmental issues including autism during all baby well visits. Then, a formal screening for ASD should take place at 18 months and again at 24 months. If there are signs, your child should be referred to a specialist right away. The earlier intervention programs begin, the greater the potential benefits.
