Where it falls apart is the definition of "need" in this scenario. It's a problem in that we use the same word for multiple degrees of severity for the same thing.
An infant could conceivably live without their parents, say they were raised by wolves. It's a fantastic example, but it could conceivably happen. Where is the mother in the equation? Any caretaker is enough to fulfill the role, the mother is not a critical factor when considering need.
Compare this to a human fetus in the womb. The mother is inextricably linked to the viability of the fetus, at least until the point of fetal viability. A fetus could not survive without the direct consent of the biological mother to continue the pregnancy. Until that viable point, the fetus is an element of the mother, totally dependent on reaching that point in order to survive independently.
This is why pro choice people who are willing to restrict abortion often stop at fetal viability as a red line. Before that, they're completely an element of the mother, completely discrete from even an infant which has autonomy beyond that of leeching nutrients from a host. Babies have an immense amount of independence if you really think about it.