The potential biological effects of in utero radiation exposure of a developing fetus include prenatal death, intrauterine growth restriction, small head size, mental retardation, organ malformation, and childhood cancer. The risk of each effect depends on the gestational age at the time of exposure, fetal cellular repair mechanisms, and the absorbed radiation dose level. A comparison between the dose levels associated with each of these risks and the estimated fetal doses from typical radiologic examinations lends support to the conclusion that fetal risks are minimal and, therefore, that radiologic and nuclear medicine examinations that may provide significant diagnostic information should not be withheld from pregnant women. The latter position is advocated by the International Commission on Radiological Protection, National Council on Radiation Protection, American College of Radiology, and American College of Obstetrics and Gynecology. However, although the risks are small, it is important to ensure that radiation doses are kept as low as reasonably achievable.