As scientists struggle to understand the causes of autism, a potential new pattern has emerged: The condition is associated with induced or augmented labor, according to a new study.
Induction means stimulating contractions before spontaneous labor begins. Augmentation means helping contractions become stronger, longer or more frequent. Both of these methods of expediting deliveries have helped mothers who have health conditions that could be detrimental to them or their child.
The researchers did not prove that these treatments cause autism. Women should not read the new study, which is published in the journal JAMA Pediatrics, and decide against expediting labor on that basis, said Simon Gregory, researcher at Duke University Medical Center and lead author of the study.
"It’s a decision between them and their healthcare provider," Gregory said, but the data do not "outweigh the risks that would come with just not wanting to be induced or augmented at all, because then you’re the placing the mother and the infant’s life at risk."
Autism spectrum disorders are developmental conditions characterized by social, communication and behavioral difficulties.
About 1 in 88 children has a diagnosed autism spectrum disorder, according to the latest estimate from the Centers for Disease Control and Prevention.
Although there is evidence that genetics plays a role, environmental factors may also be at play in altering normal development. A recent study of twins found that susceptibility to autism can increase in prenatal and early postnatal environments.
The study looked at more than 625,000 records of children's birth and education from North Carolina. Researchers obtained information on the demographics of both parents, the mother's medical history age at pregnancy, and infant health.
Although this is a large sample size, study authors could not control for every variable that might have influenced the results. They did not have information about paternal age, for example, or what medications the mothers were taking. Researchers also did not obtain data about where on the autism spectrum the children fall in this study.
Researchers found a strong link between treatments to expedite labor and males who had autism; for females, less so.
Male infants born in deliveries in which labor was both induced and augmented were 35% more likely to have autism than those whose mother did not have either of these treatments. For induction alone, risk was elevated 18%. For augmentation alone, risk went up 15%.
The risk to females was not significantly elevated when labor was both induced and augmented, or induced alone. The likelihood of autism went up with augmentation alone, 21%.
"The risk is modest but significant, particularly considering that this is a potential risk factor many pregnant women may be exposed to during labor," according to a statement from Autism Speaks, a leading autism science and advocacy organization.
The gender gap seen in the study is intriguing to scientists, Gregory said, because autism is more common in males in general - in fact, nearly five times as many boys than girls have autism spectrum disorders.
What it means, however, is unclear.
Researchers also found support for other autism risk factors that previous studies have established. Older maternal age raised the risk 30%, being first born increased risk 21%, and having a mother with gestational diabetes upped the risk by 24%.
They did not find any increased risk for children born in Cesarean sections compared to vaginal births.
This data does not demonstrate that induced or augmented labor causes autism. It only shows an association; scientists do not yet know what explains the connection.
Gregory said there could be a number of underlying factors that this study did not directly address, including the health of the mother, drugs used to induce or augment birth, fetal stress, or other medications that the mother is taking. The act of inducing or augmenting may be to blame, but alternatively the medical and obstetric conditions around those treatments could have something to do with it, or even some other events that commonly occur to women whose labor is expedited. At this stage, no one knows.
But researchers say the underlying mechanism is worth looking into, given that expedited labor isn't rare. About 23% of births in the United States in 2008 were induced, and 17% were augmented in 2002, Gregory said.
"This is the largest study to date demonstrating an association between induced or augmented childbirth and autism, and the next step is for research to better understand the possible mechanisms behind this relationship," according to a statement from Autism Speaks.
Gregory and colleagues want to go back to these medical records and dig deeper, but also study other mothers and children going forward to see if they can figure out why they're at increased risk.
CNN's John Bonifield contributed to this report