When does the natural curvature of the spine completely occur?

The natural curvature of the spine only occurs when the newborn's spine is slightly kyphosis. With the development of head-up movement for about 3 months, 1 degree curved cervical lordosis appears, and after 6 months, the second curved thoracic lordosis appears when sitting, and the third curved lumbar lordosis appears when1year-old walks. Natural curvature refers to the physiological curvature of human spine. The human spine is not curved on the coronal plane, but on the sagittal plane. The spine has four physiological curvatures, also known as "physiological curvatures", namely, cervical curvature, thoracic curvature, lumbar curvature and sacral curvature. The normal natural curvature of the cervical spine is forward bending, but in some cases, it can be seen that the cervical spine is stiff and hairy, and the lateral X-ray film of the cervical spine can see that the curvature of the cervical spine becomes straight.