[causi]

would it be acceptable to you for me to force you to give a pint of blood every day to keep alive someone in a coma who could not live but for your blood?

Apt if the creation of the fetus wasn't your choice, but inaccurate if it was. If someone loses their balance and you grab their hand to stop them falling off a cliff, is it murder to deliberately let go before they regain their balance? Why can a person be legally compelled to sustain the life of a child after birth but not before?

[Eisenstein]

None of this has anything to do with the difference between a unviable birth and an infant.