For purposes of this particular debate, I already conceded that the fetus is a person.
But it's a person on life support, basically -- particularly before the point of fetal viability. It's a person who has no independent means of living apart from its mother's willingness to carry it in her womb.
The state cannot force a person to donate blood, or a kidney, or even force a dead person to donate his or her vital organs to save someone else's life. And correspondingly, we should not force a woman to use her womb to support a fetus's life.
Again: taking that position would mean giving the fetus more rights than a living person, and the mother fewer rights than a dead person.
A mother who failed to feed her child with breastmilk, or formula, or otherwise, or entrust the child to the care of another, would be properly charged with criminal neglect. Nothing says she has to use her breastmilk. Plenty of mothers cannot, or choose not to.