Prenatal sexual dimorphism in human pelvic tilt at the onset of fetal ossification

BACKGROUND: Sexual dimorphism in pelvic morphology has been confirmed at the onset of primary ossification. Since pelvic tilt influences spinal alignment and bipedal locomotion, determining the initial emergence of sex-specific tilt differences is crucial. Herein, we measured pelvic tilt and lumbosacral inclination angles (LSIAs) using magnetic resonance imaging in 72 human fetuses (crown-rump length [CRL], 50-225 mm; 34 males [M] and 38 females [F]). RESULTS: The cranial-caudal axis was defined from the first to third sacral vertebrae, and sex differences were assessed using multiple regression analysis including sex, CRL subgroup, and their interaction. Females exhibited greater posterior iliac tilt than males (anterior superior iliac spine-posterior superior iliac spine line: least-squares mean; [F] -19.6° vs. [M] -17.0°; p = .038). The LSIA (L1/S5-sacral promontory) was larger in males ([F] 166.3° vs. [M] 170.2°; p = .033), indicating more dorsally positioned sacra in females. Principal component analysis identified two major components representing os coxae rotation and sacral inclination to the lumbar spine, indicating independent developmental patterns. CONCLUSIONS: The findings demonstrate that pelvic sexual dimorphism includes tilt orientation from the onset of ossification, reflecting sex-specific developmental trajectories during the prenatal period.