The difference between a baby and a fetus is one of location/environment, not development. People tend to think of them as different stages with fetus coming first and baby following it (which, to be fair, is how it usually works), yet one woman can have a fetus that is actually older than a baby.

Lastly, depending on how adventurous the doctor is and how open to new experiences the mother is, you can even have a fetus with no rights who becomes a baby and inherits the rights of a person in that jurisdiction, who then turns back into a fetus who would lose their rights in some jurisdictions while retaining them in others.