Children of women who had labor induced or sped up with drugs were more likely to go on to develop an autism spectrum disorder, in a new study.
The study's lead researcher, however, said the findings don't prove inducing or speeding up - also known as augmenting - labor causes autism, and they shouldn't affect decisions to use the techniques.
"The benefits of induction or augmentation by (obstetricians and gynecologists) far outweigh the risks to maternal and fetal health," said Simon Gregory, from Duke Medicine in Durham, N.C.
About one in 88 children has an autism spectrum disorder, according to the U.S. Centers for Disease Control and Prevention.
Previous studies have found that early environmental exposures, such as pregnant women's use of epilepsy drugs and folic acid, are tied to children's risk of developing autism.
For the new study, Gregory and his colleagues wanted to examine the relationship between medically starting or quickening labor and autism, which had been looked at in smaller studies with conflicting results.
Doctors typically use drugs to induce labor when a woman's pregnancy lasts one or two weeks past her due date. For augmentation, doctors also use medicine to quicken or restart a woman's contractions.
When medically indicated, induction has been linked to fewer complications, including a smaller chance of infant death.
The researchers used information on births in North Carolina between 1990 and 1998 and linked that with education data from 1997 and 1998 and from 2007 and 2008.
Of approximately 911,000 babies, Gregory and his colleagues were able to match about 678,000 to their education records, which included a note when a child had been diagnosed with autism.
According to findings published in JAMA Pediatrics, 4,285 boys were diagnosed with autism, compared to 1,363 girls.
Of boys diagnosed with autism, about 14 percent were delivered after an induced labor and about 16 percent were delivered after augmentation. In comparison, 13 percent of boys with typical development were born after induction and 14 percent after augmentation.
Among girls, only augmentation was linked to an increased autism risk. About 16 percent of girls with autism were born after augmentation, compared to 14 percent of those with typical development.
"At this stage we're assuming it's a genuine biological phenomenon," Gregory said, adding that one plausible explanation is that the drugs delivered intravenously to the mother can cross the placenta and enter the soon-to-be-delivered child.
If that is the case, the researchers estimate that eliminating induction and augmentation may prevent about two of every 1,000 autism cases among boys.
But Gregory cautioned that the study can't prove induction or augmentation causes autism. Other events occurring at delivery, maternal health or genetic factors - which may have made induction or augmentation necessary in the first place - may contribute to the increased risk, he added.
"Because we haven't found this direct link we're not saying this should change standard clinical practices," he said.
Dr. Susan Hyman, an autism specialist at the University of Rochester Medical Center in New York, said whatever the cause, the study supports the American Academy of Pediatrics recommendation that children be screened for autism at 18 and 24 months followed by ongoing surveillance.
Hyman, who was not involved with the new research, also echoed the researchers' conclusions that more studies are needed and these findings should not change the current standard of care.
"Induction is extraordinarily common," she said. "Discuss that with your healthcare provider if you're worried about your child. Although the statistics identify an association the vast majority of children are fine and many of their lives might have been saved (by induction)."