A new study finds no link between autism and number or duration of ultrasound exams. But depth of ultrasound exams needs more study.

When pregnant women undergo ultrasound exams, does it increase their children’s chances of developing autism?

New research reported in JAMA Pediatrics offers some reassuring results.

After examining the fetal ultrasound records of 420 patients at Boston Medical Center (BMC), the authors of the study found no link between the average number or duration of ultrasound exams and the development of autism spectrum disorder (ASD).

“The study was very reassuring in that these aspects of ultrasonography seemed to be safe,” Dr. N. Paul Rosman, a pediatric neurologist at BMC and the study’s first author, told Healthline.

“There’s naturally concerns about the duration of ultrasound, how often during pregnancy a child might be studied with ultrasound, and in what trimesters a child might be studied, and none of those factors seemed to play a role in terms of being associated with autism,” Rosman said.

Rosman’s research team compared the records of 107 children diagnosed with ASD to those of 104 children diagnosed with other developmental delays and 209 children classified as having typical development.

The researchers found no statistically significant difference in the average number of fetal ultrasound exams that mothers from each group underwent.

They also found no significant difference in the average estimated duration of ultrasound wave exposure across all three groups.

However, the research team did find the average depth of ultrasound penetration was greater in mothers of children with ASD, compared to mothers of children with typical development.

Rosman described this finding as “interesting.”

But he cautioned that more research is needed to learn if this association is replicated in larger studies and what its clinical significance might be.

Rosman’s research team relied on fetal ultrasound records to retrospectively study different parameters of ultrasound exposure.

For example, they used the timestamps on ultrasound screenshots from patients’ records to estimate the amount of time that patients were exposed to ultrasound waves.

When they calculated the duration of each ultrasound scan, they used the timestamp on the first screenshot as the start time and the timestamp on the last screenshot as the end time.

As a result, they didn’t capture ultrasound exposure that might have occurred before the first screenshot or after the last screenshot was taken.

This is one of the limitations of the study raised by Dr. Jacques Abramowicz, a member of the Bioeffects and Safety Committee of the International Society of Ultrasound in Obstetrics and Gynecology (ISUOG).

“If the first image is at 10:00 and the last one was at 10:30, does that mean that this patient had an ultrasound that lasted 30 minutes? No it doesn’t,” Abramowicz told Healthline.

“It could be 20 minutes or it could be 2 hours because maybe she had the scan for an hour before the first image was taken,” he added.

Abramowicz also raised some methodological concerns about how the authors of the study assessed and interpreted the mean depth of ultrasound penetration.

In the discussion section of their paper, the authors suggest that deeper ultrasound penetration might injure the brains of fetuses, increasing their risk of developing ASD.

In support of this theory, they reference past animal studies in which prenatal ultrasound exposure appeared to negatively affect the development of rodents’ brains and their behavior.

However, Abramowicz argued that the depth of ultrasound penetration is not a reliable measure of how deeply ultrasound penetrates fetal tissues.

“On the side of [ultrasound screenshot], the depth of penetration is how deep the ultrasound wave penetrates into the mother’s body,” he said.

One of the factors that influence this parameter is how large the mother’s abdomen is.

Rosman and colleagues reported the proportion of mothers in each group that was clinically obese, but they did not report mothers’ average body mass index.

This makes it harder to interpret their findings, Abramowicz said, especially since the difference in average depth of penetration between groups was less than 1 centimeter in each trimester.

“Furthermore, we don’t know from the image what part of the fetus was at the focal point. Was it the left hip? The right foot? The abdomen? Or was it the brain?” Abramowicz said.

“If it were the brain, you might say, ‘Aha, that’s the problem!’ But we don’t know that. There’s no way of knowing that,” he pointed out.

In the future, Rosman would like researchers to conduct a prospective study of ultrasound exposure and ASD among a larger number of participants from multiple medical centers.

This could potentially shed more light on the association that his team found between depth of ultrasound penetration and ASD.

Experts believe that multiple factors could potentially contribute to the development of ASD, a group of conditions that has increased in prevalence in recent decades.

“There is growing evidence that ASD often results from a combination of genetic predisposition and environmental factors,” M. Daniele Fallin, PhD, director of the Wendy Klag Center for Autism and Developmental Disabilities, told Healthline.

“I am hopeful that we will be able to identify modifiable risk factors that can guide actionable guidelines for mothers, parents, obstetricians, and pediatricians in the future,” she said.

In the meantime, Fallin encourages pregnant patients to seek support from an obstetrician who can provide prenatal care and consultation.

As a routine part of that care, the American College of Obstetricians and Gynecologists encourages patients to undergo at least one standard ultrasound exam during pregnancy.

“It’s very helpful to get a precise dating of the pregnancy, to look for the number of fetuses, to look for the normal growth of the fetus, and to look for abnormalities in the fetus,” Abramowicz explained.

He noted that ultrasound exams should only be performed when medically indicated, by a trained professional following the ALARA principle, or “as low as reasonably achievable.”

Under this principle, the professional conducting the scan should use the lowest ultrasound energy required for the shortest amount of time needed to obtain diagnostic images.

“If an ultrasound is performed for a medical indication by a professional person who respects the ALARA principle, then the risk is, as far as we know, minimal — if it exists at all,” Abramowicz said.

“Clearly the balance is in favor of the benefits because on the other side, there’s really nothing proven,” he said.