If it is true that undocumented residents in the US are not “subject to the jurisdiction thereof”, as the 14th Amendment states, then the same legal logic applies to a fetus in the United States. A fetus, by definition, is a stateless undocumented non-citizen resident and would not be subject to the jurisdiction thereof until after it is born, per the Constitution.
It’s also worth noting that the basis for Plyler v Doe is that illegal immigrants are still “subject to the jurisdiction thereof”. If undocumented immigrants and fetuses are not subject to the jurisdiction thereof, then the basis for Plyler falls and any legal precedent for equal protection of rights for undocumented people also falls.