On average, women face a 1 in 88 chance of having a child with autism; the results suggest that obesity during pregnancy would increase that to a 1 in 53 chance, the authors said.
The study was released online Monday in Pediatrics.
Since more than one-third of U.S. women of child-bearing age are obese, the results are potentially worrisome and add yet another incentive for maintaining a normal weight, said researcher Paula Krakowiak, a study co-author and scientist at the University of California, Davis.
Previous research has linked obesity during pregnancy with stillbirths, preterm births and some birth defects.
Daniel Coury, chief of developmental and behavioral pediatrics at Nationwide Children's Hospital in Columbus, Ohio, said the results "raise quite a concern."
He noted that U.S. autism rates have increased along with obesity rates and said the research suggests that may be more than a coincidence.
More research is needed to confirm the results. But if mothers' obesity is truly related to autism, it would be only one of many contributing factors, said Coury, who was not involved in the study.
Genetics has been linked to autism, and scientists are examining whether mothers' illnesses and use of certain medicines during pregnancy might also play a role.
The study involved about 1,000 California children, ages 2 to 5. Nearly 700 had autism or other developmental delays, and 315 did not have those problems.
Mothers were asked about their health. Medical records were available for more than half the women and confirmed their conditions. It's not clear how mothers' obesity might affect fetal development, but the authors offer some theories.
Obesity, generally about 35 pounds overweight, is linked with inflammation and sometimes elevated levels of blood sugar. Excess blood sugar and inflammation-related substances in a mother's blood may reach the fetus and damage the developing brain, Krakowiak said.
The study lacks information on blood tests during pregnancy. There's also no information on women's diets and other habits during pregnancy that might have influenced fetal development.
There were no racial, ethnic, education or health insurance differences among mothers of autistic kids and those with unaffected children that might have influenced the results, the researchers said.
The National Institutes of Health helped pay for the study.