Why does the adult vertebral column have fewer vertebrae than that of a newborn?

There are 5 groups in the vertebral column. In order they are called

1) Cervical

2) Thoracic

3) Lumbar

4) Sacrum

5) Coccyx

The 4th group contain 5 bones and once you are in the adult phase, roughly at about 26 years, the 5 bones fuses into one bone which shrink the number of bones the vertebra column have but but does not lose in size.

not a 100 % sure hope this helps

c- dynamic stretching