Growth Throughout the Body
Organisms do not grow evenly.
Different parts of the body develop faster than others.
Sometimes, all parts grow evenly, but during times of growth, it is mostly faster in some places than others.
When a baby is going through development in the womb, it is the brain that grows faster than anything else. This is because a large and well-developed brain will give the baby a higher survival advantage, and the ability to learn a wide range of things. Also, when a baby is born, its head takes up 1/3 of its entire body, and so it is clear that brain develops at a very early age.
In growth spurts, the rest of the body gets a chance to catch up a litte.