Purpose <p>To evaluate the changes and influencing factors of the lumbosacral curvature in the sagittal plane before and after birth based on MRI.</p> Methods <p>Cross-sectional study. Normal 63 fetuses and 100 children who underwent lumbar MRI scans from July 2021 to July 2023 were enrolled in the Imaging Department of Shandong Provincial Hospital. Groups 1–4 were the Fetal, Pre-walking, Walking, and Post-walking groups. Parameters: lumbar lordosis Angle (LLA), lumbosacral lordosis size (d) = 180°- lumbosacral lordosis Angle (LSLA), sacral tilt (ST), sacral horizontal inclination Angle (SIA). Analysis of variance (ANOVA) and Bonferroni correction were used to compare the parameters among different groups. Independent sample t-test was used for gender differences. Pearson or Spearman correlation analysis was used.</p> Results <p>In all fetus and children: LLA were 20.5 ± 9.1°, 22.0 ± 7.6°, d were 6.0 ± 2.3°, 28.6 ± 6.1°, ST were 12.5 ± 19.4°, 41.6 ± 5.7°, SIA were 19.5 ± 17.8°, 27.4 ± 6.7°. The differences in all parameters were statistically significant. LLA, d, and ST of girls were higher than those of boys in Group 4; There were significant differences in LLA between Group 2 and 4, in d between Group 2 and 3、Group 2 and 4 in boys. There were correlations between LLA and ST, LLA and SIA, d and ST, ST and SIA in fetuses; between age and parameters, and between parameters in children.</p> Conclusion <p>Body posture and walking activity have important effects on lumbosacral curvature.</p>