10 weeks after the last menstrual period is generally accepted the embryonic period. Prior to this time teratogens can cause severe defects, with paial to complete absence of organ structures, depending upon the stage of development when the teratogen was present. The case here is of X rays exposure and its accepted that 5-10 rads will have no significant deleterious effect on the fetus sincethe threshold for gross fetal malformations is more likely to be 0.2 Gy (20 rad).