New York, Sep 2 : Exposure to diagnostic ultrasound in the first trimester of pregnancy is linked to increased autism severity, say researchers.

The greatest link is among foetus with certain genetic variations that can lead to autism, the study said.

"For kids with autism who have known genetic problems, ultrasound could be an environmental stressor that pushes them to having worst outcomes" Sara Webb from the University of Washington, was quoted as saying to komonews.com. 

Previous research showed ultrasound exposure in-utero caused mice to exhibit autistic-like symptoms.

However, the new study analysed the variability of symptoms among kids with autism, not what causes autism, the researchers stated.

"Right now, there is no evidence to suggest that ultrasound itself is a risk for autism. But, it does suggest that there are pregnancies, where the foetuses are vulnerable," Webb said. 

Diagnostic ultrasound should only be used for medical necessity, according to recommendations from US Food and Drug Administration (FDA).

"I believe the implications of our results are to bolster the FDA guidelines," said Pierre Mourad, Professor at the University of Washington. 

However, the study found no link on the effect of ultrasound on the second and third trimester, he said.

For the study, published in Autism Research, the team used data from 2,644 families among 12 research sites across the US.