Mental retardation in children exposed in utero to the atomic bombs in Hiroshima and Nagasaki.

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Abstract
Subjects who were exposed in utero to the atomic bombs in Hiroshima and Nagasaki, along with suitable controls, are examined annually at the Atomic Bomb Casualty commission. Of the 1613 subjects in the study sample, 30 have gross mental retardation. Within 1500 meters from the hypocenter the prevalence of mental retardation is 5 times as high as for more distal subjects, and 6 to 15 weeks gestation was the most sensitive period. Even when subjects with possible " explanations" for their retardation are excluded, the pattern of differences remains the same. All but 2 of the retarded subjects had smaller than average heads and for those who were within 1500 meters this effect is accentuated.