See, the only time it is ok to kill it is via abortion. IAW Johnson v Florida, the mother can be charged with manslaughter for delivering drugs to a minor if she's a coke addict. In the Peterson case, the father was convicted of killing the person of his unborn child. It seems the law overall states the child is a child - unless the mother chooses to have a doctor suck it into a blender, then it has no individual rights as a person.
We have no coherent, unified legal definition, so I must go to common sense and science. The science says the fetus/baby is a completely separate human being, and it doesn't just magically turn into one at 24 weeks - it always was one. This makes it far different than a hairy pimple. Meanwhile, the common sense of personal responsibility says that the mother willingly placed it there and is therefore responsible - tacitly accepted, as Sappy said - for that child until birth.
A few legal scholars disagreed, and rejected the child's rights as a person until the third trimester, where they seemed to invoke the magical, fairy-tale thought that a wand is swept over the mother in her sleep one night and turns the pimple into a person, with rights and all. I am not a believer in magical fairy wands, so I go with the science.