Objective: To evaluate whether abdominal–pelvic radiotherapy for childhood cancer impairs uterine function and increases the risk of pregnancy complications and adverse pregnancy outcomes. Design: Nested cohort study. Setting: Not applicable. Patient(s): Childhood cancer survivors previously exposed to abdominal–pelvic radiotherapy (RT-exposed CCSs) as part of their treatment for childhood cancer. Intervention(s): Radiotherapy-exposed CCSs (n = 55) were age- and parity-matched to nonirradiated CCSs (non–RT-exposed CCSs; n = 110) and general population controls (n = 110). Main Outcome Measures: Uterine volume, pregnancy complications, and pregnancy outcomes. Result(s): Among nulligravidous participants, median (interquartile range) uterine volume was 41.4 (18.6–52.8) mL for RT-exposed CCSs, 48.1 (35.7–61.8) mL for non–RT-exposed CCSs, and 61.3 (49.1–75.5) mL for general population controls. Radiotherapy-exposed CCSs were at increased risk of a reduced uterine volume (<44.3 mL) compared with population controls (odds ratio [OR] 5.31 [95% confidence interval 1.98–14.23]). Surprisingly, the same was true for non–RT-exposed CCSs (OR 2.61 [1.16–5.91]). Among gravidous participants, RT-exposed CCSs had increased risks of pregnancy complications, preterm delivery, and a low birth weight infant compared with population controls (OR 12.70 [2.55–63.40], OR 9.74 [1.49–63.60], and OR 15.66 [1.43–171.35], respectively). Compared with non–RT-exposed CCSs, RT-exposed CCSs were at increased risk of delivering a low birth weight infant (OR 6.86 [1.08–43.75]). Conclusion(s): Uterine exposure to radiotherapy during childhood reduces adult uterine volume and leads to an increased risk of pregnancy complications and adverse pregnancy outcomes. Preconceptional counseling and appropriate obstetric monitoring is warranted.