Time for birth?
If a baby waited until its brain was fully developed before birth, it would be in the womb for nearly two years. By then, its head would be too large to fit through its mother's pelvis. So there is a 'trade-off', which has enabled humans to keep both their large brains and upright posture: we are born with underdeveloped brains. Although we are born with most of our brain cells, the connections between them are not completed until the age of two.