This chapter discusses the risk of embryo-fetal irradiation during pregnancy. In pregnant patients undergoing radionuclide examinations, radiation may reach the embryo-fetus by means of the penetrating gamma rays and X-rays emitted by radionuclides concentrated in maternal organs or the placenta, or by radionuclides taken up by the fetus after they cross the placenta. Physicians faced with a pregnant woman who requires radiation therapy for cancer may be inclined to advocate abortion for fear of possible injury to the fetus. The chapter also discusses specific cancers complicating pregnancy that may require radiation therapy. These cancers are usually divided into extrapelvic and pelvic. The chapter describes the most common types: breast cancer, Hodgkin's disease (extrapelvic), and cervical cancer (pelvic). Radiation exposure has been shown to increase the risk of cancers in both adult and children.