This is the normal fetal spine. It is normal because all ossification centers are present and in the correct anatomical location, there is asymmetry between all vertebra, the spine shows slight convex and concave shape, the sacrum is pointed posteriorly and present.
Here is an abnormal spine that is demonstration a hemivertebrae. This is a condition where half of the vertebra in the spine does not form. The cause of a hemivertebrae is unknown however some suggest its probable cause is lack of blood supply to the vertebrae. Sonographic assessment will demonstrate a "kink" in the spine which makes the contour look off. You may also notice a wedge shaped vertebra only on a single side. Hemivertebrae can express itself as kyphosis, scoliosis and lordosis