Wouldn't this defeat the purpose of playing a mother raising a child
I don't know that there's one "the purpose" for playing a mother raising a child. You'd probably need to ask the OP what the purpose is in this case.
But, that frames the following question well - is the player's purpose in this compatible with the purposes of others playing at the table?
I don't know how much of the purpose is to actually play raising a child - that is a very long, very slow set of processes, for which we have few mechanics in the game. Raising a kid takes years, but our rules are for rounds
I would suspect, "play a character with a major obligation that constrains their action," or "play a character with an important emotional bond unrelated to adventuring" are likely purposes. These are not necessarily at odds with having the kid removed from the action, or transformed after a little while. After "protect the baby" gets old, that baby isn't really helping the story develop. The issue becomes a constant, not evolving much on the timescale of adventures.
See, for example, romances in most fantasy stories. Stable, healthy relationships are generally avoided in long-term stories, mostly because they aren't dynamic story elements. Meaning, basically, they are boring. Eventually, stable romances need to be shaken up to be lively elements. We should expect the same for the baby.