Although ultrasound during pregnancy is used extensively, there is little p
ublished on adverse fetal effects. We undertook a cohort study including me
n born in Sweden from 1973 to 1978 who enrolled for military service. We es
timated relative risks for being born left-handed according to ultrasound e
xposure in fetal life using logistic regression analysis. Eligible for the
study were 6,858 men born at a hospital that included ultrasound scanning i
n standard antenatal care (exposed) and 172,537 men born in hospitals witho
ut ultrasound scanning programs (unexposed). During the introduction phase
(1973 to 1975) there was no difference in left-handedness between ultrasoun
d exposed and unexposed (odds ratio = 1.03, 95% confidence interval (Cl) =
0.91 to 1.17). When ultrasonography was offered more widely (1976 to 1978),
the risk of left-handedness was higher among those exposed to ultrasound c
ompared with those unexposed (odds ratio = 1.32, 95% CI = 1.16 to 1.51). We
conclude that ultrasound exposure in fetal life increases the risk of left
-handedness in men, suggesting that prenatal ultrasound affects the fetal b
rain.