Becoming a parent involves not only bearing a child but adopting a new identity, an expected identity, and new behavior. If soon after birth one asked a mother or a father if they felt like a parent they would say "no," and likely give the same answer several months later since they had not yet fully incorporated parenting into their self-image and range of behavior. The young parent must battle to safeguard an area of "self" against the demands of their baby-intruder.
Then, years later, the opposite happens when the parent must re-discover and re-mold their sense of who they are when their child leaves the home, depriving them of the caretaking role.
Slowly, throughout life, multiple behaviors and new functions are added to the expanding sense of who one is: the being as sexual, the being as worker, and the being as parent, all intertwined, similar to how the biological body metabolizes food to make it usable before incorporating its nutrition within itself.