A female cancer survivor, having suffered malignant lymphoma during childhood and received radio‐ and chemotherapy, became pregnant. Her uterus was extremely small and in the 22nd week of gestation, acute uterine contractions occurred, leading to preterm delivery and resulting in the death of the infant. Pelvic irradiation in childhood was considered to have caused a small uterus and thus brought on preterm delivery. The younger the patient, the more vulnerable the uterus is to irradiation. The exposure dose to the uterus in pediatric cancer patients should therefore be reduced, especially in younger patients. The cooperation of pediatric cancer specialists and obstetricians is necessary to preserve the future fertility of female cancer survivors.