Autism spectrum disorders (ASDs), including classic autism, Asperger syndrome, pervasive developmental disorder, childhood disintegrative disorder, and Rhett syndrome, affect an estimated 1% of children in the United States and United Kingdom, with boys 4 to 5 times more likely to be affected than girls. The etiology of ASDs is multifactorial, with environmental exposure possibly having a role. Speculation has grown that obstetric ultrasound is one cause, but whether it is a direct cause or a coincidence is still unclear.
A 10-fold increase in the prevalence of ASDs has occurred in the last 40 years. Whether this increase is due to diagnostic criteria, improved diagnosis, increased public and professional awareness, or inclusion of milder cases remains controversial. One possible cause is the use of ultrasound during pregnancy. The question becomes one of a parallel increase in the prevalence of ASDs against a similar increase in the use of ultrasound. Although several studies have hypothesized that this link is present, others posit that the increased use of cell phones, personal computers, frozen food, television, and the like may also be causative factors. Another consideration is the increased use of ultrasound in pregnant women with disorders possibly affecting the health of the fetus (eg, bleeding, diabetes, small-for-gestational-age fetus).
Although ultrasound is considered safe, bioeffects in humans have been reported. However, most studies have shown no association between routine prenatal exposure to ultrasound and a child’s school performance or deficits in attention, motor control, perception, vision, and hearing. Non–right-handedness seems to be an effect in boys, but a cause-effect relationship, if it exists, is weak.
The potential harmful effects of in utero exposure to diagnostic ultrasound were investigated in 2 recent studies. Outcomes were compared in 37,505 pregnant women who underwent routine ultrasound examinations versus examinations for specific indications at less than 24 weeks’ gestation. The incidence of adverse outcomes for children’s physical and cognitive development was identical in both groups. A World Health Organization analysis of 61 publications found only a weak association between ultrasound exposure and non–right-handedness in boys. A major problem with current studies is that details of the ultrasound equipment used, exposure parameters, or maternal risk factors were not included.
In the human fetus, migration of brain neurons from proliferative zones to their final destination occurs at 6 to 11 weeks’ gestation but continues until 32 weeks. Ultrasound is possibly one external factor affecting this process. Syndromic autism, associated with multiple congenital anomalies, is due to an early insult, whereas the injury for nonsyndromic autism or an isolated ASD occurs at 8 to 24 weeks. In children with ASDs, altered brain growth, especially increased brain size, a decrease in the number of neurons, and alterations in neuronal patterning and cortical connectivity have been reported. These factors suggest that the neuronal-cortical organization is disturbed, resulting in information alterations at many levels. No clear pathologic alterations have been shown at the cellular level. If an external factor is involved, the actual insult probably occurs early during neurodevelopment.
Environmental factors connected with ASDs include parental age, exposure to infections, toxins, and other biologic agents, particularly vaccines, and prenatal ultrasound, with the latter 2 factors given a great deal of attention recently. In a large meta-analysis of 60 possible factors, those associated with autism risk were advanced parental age at birth, maternal prenatal medication use, bleeding, gestational diabetes, being first born, and having a mother born abroad. Among numerous perinatal factors were abnormal presentation, umbilical cord complications, fetal distress, birth injury or trauma, multiple birth, and various labor complications. Preterm neonates have a higher prevalence of ASDs. The issue of vaccines as cause for the increased prevalence of ASDs has been disproved by many scientific studies.
No independently confirmed peer-reviewed published evidence has confirmed a cause-effect relationship between in utero exposure to clinical ultrasound and development of ASDs. Because ultrasound has thermal and mechanical effects, its use should be restricted to medical indications, by trained professionals, for as short a period and at as low an intensity as possible.