A Duke University study suggests that autism might be linked with inducing and/or speeding up labor for pregnant women, especially if the baby is male.
The preliminary findings need investigating, since labor is induced in increasing numbers of U.S. women, researchers and other autism experts say.
It's possible, they say, that labor-inducing drugs may increase autism risk, or that the problems that lead doctors to start labor explain the results.
These include diabetes and fetal complications, which have also been linked to autism, though like most autism research, conclusive answers are elusive.
The authors reiterate that the results shouldn't lead doctors to avoid inducing labor or speeding it up since it can be life-saving for mothers and babies.
Duke's Simon Gregory said, "We haven't found a connection for cause and effect. One of the things we need to look at is why they were being induced in the first place."
A government-funded study published online Monday in JAMA Pediatrics suggests 1 in 5 U.S. women have labor induced – twice as many as in 1990.
Smaller studies suggested a possible tie between induced labor and autism, but the new research is the largest to date, involving more than 600,000 births.
The researchers examined eight years of North Carolina birth records, and matched 625,042 births with public school data from the late 1990s through 2008.
Information on autism diagnoses did not specify whether cases were mild or severe. Labor was induced or hastened in more than 170,000 births.
Overall, 5,648 children developed autism, three times as many boys as girls.
Among autistic boys studied, almost one-third of the mothers had labor started or hastened, versus almost 29 percent of the boys without autism.
The differences were less pronounced among girls. The strongest risks were in boys whose mothers had labor started and hastened, results showed.
They were 35 percent more likely to have autism.
Among girls, those born after labor was accelerated were 18 percent more likely to have the disorder than girls whose mothers had neither treatment.
Oxytocin and prostaglandins are among the methods used to start or speed up labor, but the Duke study doesn't identify any specific medications.
Autism affects about 1 in 88 U.S. children. Causes are uncertain but experts believe it probably results from a combination of genetics and other factors.
These may include mothers' illnesses and medication use while pregnant, fathers' age at conception, and problems affecting the fetus during childbirth.
All of those factors, like induced labor, have been previously suggested and theorized but not proven in previous research on the controversial topic.