Objective To report normative data on uterine volume and endometrial thickness in girls, according to pubertal stages; to evaluate factors that affect uterine volume; and to compare transabdominal ultrasound (TAUS) and magnetic resonance imaging (MRI). Design Cross-sectional study of a nested cohort of girls participating in The Copenhagen Mother-Child Cohort. Setting General community. Patient(s) One hundred twenty-one healthy girls, aged 9.8–14.7 years. Intervention(s) None. Main Outcome Measure(s) Clinical examination, including pubertal breast stage (Tanner classification: B1–B5). Uterine volume: ellipsoid TAUS (n = 112) and 3-dimensional TAUS (n = 111); ellipsoid MRI (n = 61). Endometrial thickness: TAUS (n = 110) and MRI (n = 60). Result(s) Uterine volume and endometrial thickness were positively correlated with pubertal stages; e.g., ellipsoid TAUS: r = 0.753, and endometrium TAUS: 0.648. In multiple regression analyses, uterine volume was associated with the number of large follicles (TAUS >5 mm) (Beta 0.270); estradiol (E2) (Beta 0.504); and height (Beta 0.341). Volumes from ellipsoid vs. 3-dimensional TAUS were strongly correlated (r = 0.931), as were TAUS and MRI: ellipsoid volume (r = 0.891) and endometrial thickness (r = 0.540). Uterine volume was larger in TAUS compared with MRI; mean difference across the measured range: 7.7 (5.2–10.2) cm3. Agreement was best for small uteri. Conclusion(s) Uterine volume and endometrial thickness increased as puberty progressed. Circulating E2 from large follicles was the main contributor to uterine and endometrial growth. The TAUS and MRI assessments of uterus and endometrium were strongly correlated.