The best guidance I have found regarding the imaging of pregnant or potentially pregnant women with x-rays is from this ACR Practice Guideline.
From the Guideline:
“If a patient can reliably answer that 1) she cannot be pregnant (for example, she is not sexually active, or she is using an effective form of birth control, or she is biologically incapable of conceiving) and that 2) she had a recent complete menstrual period, then it is reasonable to proceed with a medically indicated diagnostic X-ray test of the abdomen or pelvis.”
Two sample forms are included at the bottom of the ACR guidance. These forms could be used to draft facility-specific consent forms. The first could be used to document a negative response to pregnancy, the second for performing procedures on known pregnant women.