Brain damage among individuals exposed prenatally to ionizing radiation: a 1993 review.

Mental retardation as a result of prenatal exposure to ionizing radiation is not a common phenomenon when compared to the incidence of cancer, but it has nevertheless been well-documented. This article describes results from studies of individuals who were exposed prenatally to radiation in Hiroshima and Nagasaki. The critical time of exposure, when the most significant damage was done, was during the 8th-15th week of gestation, with a lesser effect at 16-25 weeks. Individuals in the study were assessed by measurement of an intelligence quotient and by examination of school performance. Studies show that the period of 8-15 weeks of gestation coincides with a key time for neuronal cell migration in the developing brain. There is continuing investigation of the mechanism of this migration and how it might be disrupted by ionizing radiation.