Antenatal ultrasound is unlikely to increase the risk of autism spectrum disorders (ASD), according to a study published Sept. 1 in the Journal of Autism and Developmental Disorders.

According to principal investigator Judith K. Grether, PhD, from the California Center for Autism and Developmental Disabilities Research and Epidemiology, California Department of Health Services, and colleagues, with an increasing number of children being diagnosed with autism, “there has been considerable speculation about environmental factors contributing to that increase.”

Antenatal ultrasound was introduced into obstetrical practice back in the 1970s and the performance of multiple ultrasound exams is now a standard means to visualize fetal growth and development, the authors said, which means there has been an increase in the number and duration of fetal exposures to ultrasound.

The authors compared affected singleton children and control children born between 1995 and 1999, who were enrolled in the Kaiser Permanente healthcare system. Thirteen percent of children with ASD from simplex families and 12.5 percent of control children had no exposure to ultrasound exams during gestation. Of those children exposed to ultrasound, the majority of exams were conduced in the second trimester—77.9 percent of cases and 79.2 percent of controls had exams during this time. In the first and third trimesters, approximately 28 percent of both cases and controls had at least one scan.

The researchers adjusted local regression models for maternal education, age, gestational age, birth hospital, birth year and gender. In models with ultrasound frequency treated as a continuous variable, no significant increased risk of ASD was observed with increasing the number of ultrasound exams during the complete gestation, or during any trimester.

Also, by gender, there was no increased risk of ASD with increasing numbers of ultrasound exams--except for females in the second trimester.

In models that treated frequency of exams as a variable, there was no significant elevation in risk with increasing numbers of exams either throughout the entire gestation period or in one trimester.

“The predominantly null finding of this investigation of fetal exposure to antenatal [ultrasound] and risk of autism spectrum disorders may provide assurance that antenatal ultrasound examinations, when performed in a clinical setting according to established guidelines, do not appear to put fetuses at increased risk for developing ASD,” concluded the authors. “Given the substantial size of our study, it is unlikely that an association between [ultrasound] exposure and ASD would be seen in larger numbers of cases and controls had been included, although further studies among girls are indicated.”