For many families, the quest for the causes of autism has grown more urgent with the news that the estimated prevalence of autism grew by 23% from 2006 to 2008, according to a Centers for Disease Control and Prevention report out last week.
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
In this case, immune attack in womb could be to blame
As a baby, Zachary �Herbie� White seemed like any other child.
At 15 months, he would blow kisses and wave goodbye to his grandparents. Soon, however, Herbie began losing many of these new toddler skills. He stopped waving goodbye. He didn�t talk or even look up when people entered the room.
Like one in 88 American children � and one in 54 boys � Herbie was diagnosed with an autism spectrum disorder.
Yet research suggests that the roots of Herbie�s autism may have taken hold long before he began losing interest in the people around him. While doctors may never be able to prove the cause of Herbie�s disorder with 100% certainty, research suggests that his autism may be the result of an immune assault that occurred while he was still in the womb.
Research by scientist Judy Van de Water at UC-Davis� MIND Institute suggests that, in certain cases, mothers make antibodies � proteins that normally attack foreign invaders, such as germs � that attack proteins in their baby�s brain before birth. In a study of 316 children, she found these antibodies in 18% of the moms of autistic kids � including Herbie�s � but none of the moms of non-autistic children.
When she transferred the antibodies into pregnant monkeys, their offspring also showed signs of autism, Van de Water says. Van de Water can�t explain what causes mothers to make antibodies against their own children, or how this immune attack might cause the communication and socializing problems that characterize autism.
�They won�t say this caused autism,� says Herbie�s father, Steven White, 54. �They�re being very cautious.�
White, of Haywood, Calif., says he�s glad that he�s enrolled in Van de Water�s research, both in the hope of helping his son, who will turn 5 in May, as well as other children. �It was a huge comfort,� he says, �to think I�m doing everything I can do.�
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 4� 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 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.
