These numbers are related to fetal dose and were presented at the 2008 ARRS Meeting by Mark Bernstein, MD from NYU. This was presented within "Imaging of Traumatic Injuries in Pregnancy."
In general, <5 rads is negligible.
C spine Xray - undetectable
Chest Xray - <0.0001 rad
Pelvic Xray - 0.2 rad
Head CT - <0.05 rad
Chest CT - <0.5 rad
Abd/Pelvis Ct - 3-5 rad
Link to webpage with information on Pregnancy and Radiation Exposure