microcephaly and mental impairmentor retardation. Depending on the dose, dose rate and period of gestation, the severity of these effects may range fromclinically undetectable, or insignificant, some degree of disability, to incompatibility with intrauterine or neonatallife. Although the threshold for specific end points may be greater, the overall risk to the embryo or fetus of exposure at any time during gestation does not appear to be increased below a threshold of 50 mGy (29). 3.3 Stochastic Effects Experimental and epidemiological studies have contributed an extensive body of knowledge about radiogenic stochastic effects. The clinical outcomes are not unique to radiation. Their association with radiation has been established only by observation among a numberofirradiated populations of disease rates that increase significantly with increasing dose. It currently is not possible to unequivocally attribute specific outcomes in individuals to their exposure to radiation. Relationships between specific outcomes and prior radiation exposure are necessarily expressed in terms of probability using estimates of risk derived from epidemiologic studies of irradiated populations, and taking into account known risk factors such as age at exposure and gender. 3.3.1 Heritable Genetic Effects There is no genetic disease that is uniquely radiogenic. Increased rates of conditions associated with inherited genetic mutations have been observed amongthe progeny of experimentally irradiated organisms. However, to date there is no evidence ofstatistically significant increases in the rates of genetic diseases or conditions among the progeny of irradiated populations (30). 3.3.2 Somatic Effects Epidemiologic studies of such populations have clearly identified the major stochastic effect of ionizing radiation to be a dose-related increase in the risk of tumors, primarily malignant tumors. This association has been demonstrated statistically for all cancers 11