According to a study recently published in the Journal of the American Medical Association (JAMA), researchers have found that increased depth of prenatal ultrasound exams has no associatation with late diagnosis of autism spectrum disorder (ASD).  

"A combination of genetic predisposition and environmental exposures likely contribute to a heightened risk for ASD," wrote lead author of the study N. Paul Rosman, MD, from Boston Medical Center and a pediatrics and neurology professor at Boston University. "This study assessed prenatal ultrasonography as a possible environmental exposure contributing to the increasing incidence of ASD."

Children were recruited based on prenatal care medical records obtained from Boston Medical Center, which included 328 boys and 92 girls at an average age of 6.5 years old. Of the 420 children, 107 had ASD, 104 controls had developmental delays and 209 other controls experienced typical development.

Prenatal ultrasound frequency, timing, duration and strength were analyzed from each participant's prenatal records. According to study methods, ultrasonographic exposure was calculated by the number and timing of scans each child received, the duration of their exposure, mean strength and the time of Doppler, 3D and 4D imaging.

Researchers found children who developed ASD were exposed to a greater mean depth of ultrasonographic penetration during the first and second trimesters compared to children with developmental delays or no delays at all, according to study results. Overall, no correlation between the number of scans or the duration of ultrasound exposure was found to impact late ASD diagnosis.

"As ultrasonographic depth increases, wave frequency and strength decrease, but the volume of fetal tissue exposed to ultrasonic energy increases because the wave energy dissipates in a wedge shape," the researchers wrote. "Therefore, use of greater-depth ultrasonography may be more likely to alter subependymal or germinal matrix cell migration rather than the more superficial cerebral cortical cells of the developing brain." 

The ASD group also had an overall shorter duration of scans during these two trimesters than controls with typical development. Mothers of children with ASD were more likely to be 35 years or older and to be late registrants for prenatal care. 

"Socioeconomic factors, such as access to care, are likely at play in these associations because lower socioeconomic status has been associated with an increased risk of ASD and probably with less access to prenatal care," the researchers wrote.