Two new studies add to a growing body of evidence pointing to pregnancy as a critical period in the brain changes that lead to autism.
One study, published Tuesday in Annals of Neurology, finds a four-fold increase in autism among women who had very low levels of a key thyroid hormone, called thyroxine. Researchers found the link in a study of more than 4,000 Dutch mothers and children. Doctors took blood samples from women around the 13th week of pregnancy, then followed up six years later, asking women to fill out a standard psychological checklist about the child's behavior and emotional traits.
A second study, published Monday in JAMA Pediatrics, notes that pregnant women who have their labor started or sped up artificially are slightly more likely to have autistic children.
The increased autism risk in the JAMA study likely stems from an underlying problem with the pregnancy, rather than any of the methods used to jump-start labor, says lead author Simon Gregory of the Duke Institute of Molecular Physiology.
It's possible that "infants destined to develop autism are less likely to send out the correct biochemical signals for normal progression of labor," says Tara Wenger, a pediatric genetics fellow at the Children's Hospital of Philadelphia, who wasn't involved in either study.
Authors of the JAMA study note their research doesn't definitively prove a link betwen labor induction and autism. They say doctors shouldn't change the way they manage labor and delivery based on their study, funded by the Environmental Protection Agency.
Gustavo Roman,lead author of the thyroid research and a professor of neurology at at Weill Cornell Medical Center in New York, says his study can't prove cause and effect, either. But proper thyroid function is crucial to many aspect of health. So says it makes sense that women who are pregnant, or planning to become pregnant, ask doctors to check their thyroid function and urine iodine. These women also should take prenatal vitamins that contain iodine, a key element in thyroid function, Roman said in a statement.
Pregnancy complications increase the risk of many developmental disorders, says Michael Rosanoff, associate director for public health research and scientific review at Autism Speaks, an advocacy group.
And growing number of studies now link autism to a variety of things that can compromise the health of a pregnancy, says Rosanoff, who wasn't involved in either study. Researchers are increasingly looking at prenatal risk factors for autism, because this period plays a key role in brain development. Science has ruled out vaccines as a cause of autism, he says.
But studies have found that children are at higher risk for autism if they are born early or very small; if they are in medical distress during delivery; if they have older mothers or fathers; or if they are born less than a year after an older sibling. Autism risk also goes up if a mother has diabetes or high blood pressure; is obese; is infected with rubella, or German measles; is exposed to significant air pollution during pregnancy; had low levels of folic acid; takes medications such as an anti-seizure drug called valproic acid; or makes antibodies toxic to the fetal brain.
Science has ruled out vaccines as a cause of autism, says Gregory, who notes that the original myth about autism and immunizations arose from bogus research that has since been retracted.
And while the link between autism and jump-starting labor is consistent with smaller, earlier studies, the research doesn't prove that labor induction or augmentation actually causes autism, Gregory says.
In Gregory's study, researchers analyzed the records of 625,042 North Carolina births, which were linked to school records that noted any diagnosis of autism.
Those records didn't specify where children fell on the autism spectrum, the study says.
Doctors can induce or speed up labor in several ways, such as by applying a hormone gel to the cervix or giving women intravenous oxytocin, an artificial version of a natural hormone involved in labor, Gregory says.
Women whose labor was induced were 13% more likely to have an autistic child, compared to women whose labor wasn't induced. Women whose labor was sped up were 16% more likely to have a child later diagnosed with autism, the study found. Those whose labor was both induced and augmented were 27% more likely to have an autistic child.
In comparison, the risk of autism went up 23% when the mother had diabetes, and 25% when the fetus was in distress, the study says.
There was no increase in autism risk among women who had C-sections, Gregory says.
Women shouldn't be afraid to have their labor induced, says JAMA study co-author Chad Grotegut, a Duke maternal-fetal medicine specialist. The risk of harm to a baby in distress is far greater than the modest risk of autism. Inducing labor can often reduce the risk of stillbirth, such as when a pregnancy lasts more than a week or so beyond the usual 40 weeks, says Grotegut, a Duke maternal-fetal medicine specialist. Augmenting labor, which may be done when natural labor stalls, can reduce the risk of maternal or fetal infections or post-partum hemorrhage.
But Gregory says scientists should look more closely at labor induction with oxytocin. Oxytocin, sometimes called the "love hormone," plays a key role in social behavior and reasoning.
Gregory and other researchers are investigating oxytocin to treat some of the symptoms of autism.
About one in 88 American children have an autism diagnosis, according to the Centers for Disease Control and Prevention.
Rather than one disease, autism is now regarded as a collection of conditions with similar traits but different causes, Rosanoff says. People on the autism spectrum are extremely diverse. Some are non-verbal and profoundly disabled; others have successful careers, particularly in science and technology, describing themselves as different, rather than disabled.
"Autism is so heterogeneous," Rosanoff says. "We're never going to get to the one cause."
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