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By Maggie Fox

A new study meant to show whether ultrasound scans can cause autism appears to have added to evidence that they don’t.

Women who had more ultrasound scans, or longer scans, were not any more likely to have children with autism than women who did not, the study found. But women whose ultrasound scans went deeper were slightly more likely to have children with autism.

“I think that this study is actually reassuring,” said Dr. Jodi Abbott, a specialist in obstetrics and gynecology at Boston Medical Center who helped lead the study.

“This small study reports reassuring findings that children with autism spectrum disorder were less exposed to prenatal ultrasound and that the ultrasound energy used during the examination was no different compared to children without autism spectrum disorder,” agreed Dr. Basky Thilaganathan, spokesperson for the Britain’s Royal College of Obstetricians and Gynecologists, who was not involved in the research.

Ultrasound can heat tissue, which can damage it. There have also been concerns about whether it might affect a developing fetus — especially the brain, which is fragile during early development.

“Increased temperature has known effects on the first trimester developing brain,” Sara Jane Webb and Pierre Mourad of the University of Washington, who were not involved in the study, wrote in a commentary published in the Journal of the American Medical Association’s JAMA Pediatrics.

“We know that heating and vibrating the brain is bad,” Webb said in a telephone interview.

Ultrasound can both heat and vibrate tissue.

Some studies of animals have suggested that ultrasound could affect the developing brain. Mice exposed to heavy doses of ultrasound in the womb, for instance, showed less social behavior.

And several studies have suggested that children exposed to ultrasound while in the womb are more likely to be left-handed — not anything undesirable, but an indication that the scans might affect the brain.

Plus, autism is being diagnosed more and more often.

“The prevalence of autism spectrum disorder has been increasing rapidly, with current estimates of one in 68 children affected,” Abbott and colleagues wrote.

“Simultaneously, use of prenatal ultrasonography has increased substantially.”

The Boston team looked at the case records of 420 children born at Boston Medical Center between 2006 and 2014. Their mothers got on average five ultrasounds during pregnancy.

They did not find any major differences among the 107 kids with autism, the 104 with other developmental delays and 209 normal children. Obese mothers were more likely to have children with autism, an already known fact.

“Mothers of children with autism spectrum disorder were more likely than mothers of children with typical development to be 35 years or older,” the researchers added. That’s also known.

The scans of children who later were diagnosed with autism did go a little deeper into the mother’s abdomen, the researchers found. It could be because those mothers had more abdominal fat, Abbott said.

It’s not clear what this means, outside researchers said. It’s one of those studies that leads doctors to say that more research is needed.

The American College of Gynecologists and Obstetricians recommends that women get just one or two scans during pregnancy, just to be safe, unless they are medically necessary to make sure the baby is developing normally.

Abbott said facilities performing ultrasounds for valid medical reasons calibrate their equipment to make sure the ultrasound does not heat tissue. But she said women should avoid getting ultrasounds just for fun.

“I would not recommend that a woman go for the keepsake photos,” she said.

“Ultrasound technology can be used to heat tissue.”

Webb and Mourad said the study does not completely clear ultrasound of being a potential cause of autism.

Abbott’s team did not disagree. “Ultrasound energy delivered to the fetus cannot be assumed to be completely innocuous, and the possibility exists that such biological effects may be identified in the future,” they wrote.

Many studies have found clear genetic causes of autism. Most experts say it’s likely that children with genetic predispositions to autism can develop symptoms if something else in the environment affects them. For instance, catching flu and other infections have been linked with autism risk.

Ultrasound might also be one of those stresses, Myers and Mourad said. “However, the potential for ultrasonography to act as an environmental stressor in a genetically vulnerable system remains,” they wrote in their commentary.

“We don’t know the genetic background of these young children,” Webb said.

It’s known that the developing brain is more vulnerable at certain times than others. For instance, studying the brains of babies affected by Zika virus shows that the time in which they were infected affects how serious the resulting brain damage is, and what the symptoms are.