Radiotherapy during pregnancy might cause harm to the developing fetus. Generally, pregnant women with malignant diseases are advised to delay radiotherapy until after delivery. However, this advice is not based on knowledge of the risks of radiation to the unborn child. In general, the expected radiation effects, such as mental retardation and organ malformations probably only arise above a threshold dose of 0.1-0.2 Gy. This threshold dose is not generally reached with curative radiotherapy during pregnancy, provided that tumours are located sufficiently far from the fetus and that precautions have been taken to protect the unborn child against leakage radiation and collimator scatter of the teletherapy machine; such precautions also reduce the risk of radiation-induced childhood cancer and leukaemia in the unborn child.