Mental retardation is the most prevalent abnormality to appear in prenatally exposed humans.
There is a 4% chance of occurrences per 0.1 Sv (10 rem) at 8-15 weeks of gestational age.
There is less risk with exposures at other gestational ages.
This information comes from the BEIR V report by National Academy of Sciences, 1990.
Children conceived after exposure to parents who received large exposures have
shown no effect above normal incidence of mental retardation.