 - If more than one child is born to a given set of parents, the first and last such eligible siblings will be selected for participation, in the interests of maximizing the variation of time from last exposure to conception, also to maximize the signal by including offspring closest in time to the exposure, with less likelihood of removal of damaged spermoocytes, and by including offspring furthest in time from the exposure, with less likelihood of direct exposure of the child, and maximizing cumulative parental preconception gonadal dose. 