In most cases, however, scientists can't tell parents what caused their child's autism, says Thomas Insel, director of the National Institutes of Mental Health. In large part, the causes of autism — which is likely not one disease, but a group of conditions with related symptoms — remain a mystery.
For years, scientists had only a few clues about the condition, noticing that autism is about four times as common in boys as in girls, for example.
Recently, scientists have found a number of risk factors for autism, many of which point toward problems that develop very early in life, such as during pregnancy or delivery, or even during the process of creating eggs and sperm, says Craig Newschaffer, a professor at Philadelphia's Drexel University.
To better understand causes of autism, researchers at four major universities are following 1,200 mothers of autistic children through a project called the EARLI study, or the Early Autism Risk Longitudinal Investigation.
Because researchers know that these moms are at high risk of having a second autistic child, they closely follow the women's subsequent pregnancies, testing blood, urine, hair, even vacuuming dust from the women's homes, says Newschaffer, one of the study's lead researchers.
Researchers ask pregnant women to keep lists of any illnesses, since infections during pregnancy are suspected of playing a role in autism.
Doctors can confidently reassure parents that one thing doesn't cause autism — vaccines, says Paul Offit, chief of infectious diseases at Children's Hospital of Philadelphia. Nearly two dozen studies have failed to find a link between autism and vaccines, whether given alone or in combination.
Researchers have clues to other causes:
Genes. About 15% to 20% of autistic children have a genetic mutation that causes their disorder, Insel says. Certain genetic disorders, such as Fragile X syndrome and Rett syndrome, are well-known for increasing the risk of autism.
Even when genes are the main contributor to autism, however, it's possible that most children have a unique mutation or set of mutations, says David Amaral, research director of the University of California-Davis MIND Institute.
Family history. If parents have one child with autism, the risk of having a second child diagnosed with the disorder is nearly 20%, according to a landmark study from U.C.-Davis. Among those with two autistic children, the risk of having a third is 32%, study author Sally Ozonoff says.
Environmental pollution. One California study published last year found that babies whose mothers lived near a highway while pregnant were more likely to be diagnosed as autistic.
Older parents. Both older father and mothers are at higher risk of having autistic children, Newschaffer says. Research from Israel and the Harvard School of Public Health also suggests that infertility treatments, which are more often used among older patients, are linked to a higher risk of autism.
Prematurity and low birthweight. An October study in Pediatrics found that, among babies born weighing less than about 4½ pounds, 5% had been diagnosed as autistic by age 21.
Medications. Many studies now show that a seizure treatment called valproic acid can increase the risk of autism in children exposed before birth. A single study published last year found a higher risk among children exposed prenatally to antidepressants. Using prenatal vitamins is also linked to a lower risk of autism.
Closely spaced pregnancies. In a 2011 study, children who were born less than one year after an older sibling were three times as likely to be diagnosed with autism, compared with children born three years after their mom's last pregnancy.
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