A “fetus” is a human being. Why can’t you say “baby”?
Well, you could and you can, but "fetus" means that he/she is still in the womb.
Note that we have many terms for people in various stages of their lives: fetus, preborn, baby, infant, terrible-twos, toddler, youngster, child, preteen, teenager, twenty-something; middle aged, old, elderly, and probably a few that I've left off the list.