Does prenatal sonography affect intellectual performance?

Background: Little is known about potential risks of ultrasound during pregnancy. The developing brain is susceptible to environmental influences, and prenatal ultrasound has been reported to affect cognitive abilities in animals. Methods: To assess a possible association between prenatal ultrasound and intellectual performance, we studied men born in Sweden from 1973 to 1978 who enrolled for military service from 1991 to 1996. Intellectual scores (mean +/- standard deviation = 5 +/- 2) were measured by a test battery at enrollment, and subnormal performance was defined as a score of 3 or less. We estimated differences in mean intellectual score between ultrasound-exposed and ultrasound-unexposed using linear regression, and we calculated relative risks of subnormal performance using logistic regression analysis. Results: There were 7999 eligible men born in a hospital (Malmo) that included ultrasound scanning in standard antenatal care (exposed) and 197,829 men born in hospitals without ultrasound scanning programs (unexposed). We found lower intellectual performance scores (mean difference = -0.16; 95% confidence interval = -0.21 to -0.11) and an increased risk of subnormal performance (odds ratio = 1.28; CI = 1.18 to 1.38) among ultrasound-exposed compared with unexposed. However, men born in Malmo before scanning was introduced also had lower scores, and the decrease in test scores after the introduction of ultrasound was small. Moreover, we found no differences in intellectual performance within pairs of brothers as a result of ultrasound exposure. Conclusions: This study failed to demonstrate a clear association between ultrasound scanning and intellectual performance.