To optimize the efficacy of radiotherapy for cancer patients who are pregnant, the following factors must be considered: the potential effects of the therapy on fetuses and neonates, the stage and prognosis of the mother's disease, and the possible risks to the patient of restricting cancer treatment. Malformations and mental retardation are the most serious consequences of fetal exposure to radiation that are observed after birth. The sensitivity to radiation is high from 2 to 8 weeks after conception for malformations and from 8 to 25 weeks (particularly up to week 15) for mental retardation; the risk of mental retardation declines rapidly after the 25th week of gestation. When a pregnant patient requires radiation therapy, the physician should consider fetal sensitivity to radiation in light of gestational age and the expected dose of radiation and should then calculate the risk to the fetus versus the benefits to the mother. The risk is negligible if fetal exposure does not exceed 0.1 Gy, preferably remaining below 0.05 Gy, during gestation. Furthermore, it is safest to administer radiotherapy during or after the 25th week of gestation.