Radiation-related Small Head Sizes among Prenatally Exposed A-bomb Survivors

Of 1566 individuals prenatally exposed to the atomic bombings of Hiroshima and Nagasaki, 1473 had the circumference of their head measured at least once between ages 9 and 19. Among these 1473 individuals, 62 had small heads--the circumference of the heads was two standard deviations or more below the observed specific-age-at measurement mean. Of 26 cases with severe mental retardation, 15 (58%) had small heads. Most (86%) of the individuals with small heads were exposed in the first trimester (about or = 25 weeks postovulation) or among individuals exposed at 16 weeks or more postovulation. The mean IQ values of mentally retarded cases with and without small heads were 63.8 and 68.9, respectively. No significant difference exists between these two IQ means, but both were significantly smaller than 96.4, the IQ value for individuals with small heads without severe mental retardation and 107.8, the value for the overall sample.