Exposure to diagnostic ultrasound in the first trimester of pregnancy could increase autism severity, a study published on Sept. 1 in Autism Research finds. However, the researchers did not find any effect of ultrasound when performed on the second and third trimester.

Researchers from the University of Washington Medicine, University of Washington Bothell and Seattle Children’s Research Institute studied the variability of symptoms among autistic kids. The data were from the Simons Simplex Collection autism genetic repository of 2,644 families across the United States.

The researchers assert that their study does not show that ultrasound causes autism. The findings only demonstrate that the variation of autism in kids is linked with exposure to ultrasound during the first trimester of pregnancy.

“There has been a real struggle in why there are so many kids with autism,” points out lead author Sara Webb of UW Medicine. “Where does this disorder develop from? How do kids get autism? And the second question is why are kids with autism so different from each other? This study really looks at the second question. Within kids with autism, what are some of the factors that may result in a child having a good outcome or higher IQ or better language or less severity versus a child who maybe takes more of a hit and continues to struggle throughout their lifespan?”

Further research is still needed. The research team aims to dig deeper into the links between autism severity and ultrasound exposure. Nevertheless, the researchers believe that their findings suggest improvements of current FDA guidelines.

Another study by Pierre Mourad, a professor of neurological surgery in UW, found that ultrasound exposure in-utero caused autistic-like symptoms in the offspring. However, this study was conducted on mice, not on humans. The findings were published in the journal Autism Research in 2014.