This document discusses the risks of ionizing radiation exposure during pregnancy. It notes that ionizing radiation is a known teratogen and can cause developmental effects, birth defects, and cancer risks to the fetus. The risks depend on the radiation dose and gestational period of exposure. Low doses below 5 rad are generally not considered high risk, while over 100 rad poses serious risks like mental retardation. Ultrasound and MRI are preferred over x-rays during pregnancy when possible due to radiation risks. Guidelines recommend evaluating risks and benefits of any radiation exposure case-by-case during pregnancy.