Why not simply say that the person who carried and gave birth to the child is its mother, end of?

Consider, it may be possible in the future for women, or people who were born female, to provide genetic material for insertion into an ovum in order to fertilise it. This would make that woman a father.

In other words, the physical roles of father or mother are no longer - in and of themselves -gender-specific, they simply apply to the roles played in the creation of the child.