In 1971, philosopher Judith Jarvis Thomson wrote an essay called "A Defence of Abortion" in which she argued that even if a fetus is a person with the right to life it still does not have the right to "use" its mother's body against her will. She uses a hypothetical story of a woman being kidnapped and surgically attached to a famous but ill violinist against her will in order to save his life. The argument is compelling, at least on the surface, and is still being used today, almost 50 years later, by abortion advocates to justify the killing of the unborn. I encourage you to read it here to get the full gist of her arguments.
Although it is well-reasoned and has been adopted as philosophically solid by many seeking justification for abortion, Thomson's violinist argument is fatally flawed for a number of reasons, not all of which I will address here. Greg Koukl offers a powerful and effective rebuttal here, although he has attracted criticism for applying Thomson's axioms to born children when her primary claim is limited to the physically connective relationship between mother and pre-born child.
The first objection I'll posit is that the child in the womb is not "using" her mother’s body, at least not in the sense of one person actively and intentionally violating another’s body against their will, as the violinist is. In the vast majority of abortion cases (some studies suggest 98% or more), the woman willingly engaged in the sexual act which she knew or ought to have known could result in pregnancy. She was willing to take the risk of pregnancy, thereby inviting at least the possibility of a child’s conception. Thus, the child’s presence in her womb was reasonably foreseen and is not merely an uninvited “user” of her body. She did not ask to be there, neither is she actively and intentionally “using” her mother. She is simply living, growing, and existing in the natural environment she finds herself in.
Second, the relationship between mother and pre-born child is not strictly one-way, it is symbiotic. The mother gains physiological and even emotional benefits from her pre-born child, including a buffer against cancer and healing stem cells. In these ways and others, the woman is also "using the child’s body against its will," and thus Thomson must grant the pre-born child the right to kill her mother too.
To describe the relationship of pre-born child to mother as one of one person simply using the other for their benefit is over-simplified, crass, omits any discussion of responsibility of one person toward the another, fails to acknowledge its symbiotic quality, and ignores the legitimate processes of human reproduction. It portrays pregnancy as a malicious invasion instead of a natural, healthy, and normal relationship between mother and pre-born child.
Pro-abortionists like to pit mother against child, the strong against the weak. Pro-lifers recognize the value and dignity of both parties in this most intimate of all human relationships, from the moment life begins.
Third, Thomson's violinist arguments fails to acknowledge that not all "violations of bodily autonomy" merit lethal force. For an obvious example, would I be justified in decapitating and dismembering someone to death (which is what an abortion does) if they drew a stick-man on my arm against my will? Every sane judge on the planet would say no, and I would be charged with murder and sentenced accordingly. So it comes down to which incidents of using someone else’s body justify ending the life of the user, if any. Even the woman attached to the violinist ought not to be morally nor legally permitted to slice the violinist to pieces before unhooking from him, especially if there are other, non-lethal means to disconnect from him.
Surely we can see that directly killing the violinist while still attached to the woman would be immoral, even though denying the use of her body by unhooking from him may not be. If the violinist dies after unhooking, his cause of death would be attributed to his diseased physical condition and not to the actions of the woman: a huge consideration, both morally and legally speaking. However, if she were to intentionally kill him while connected, that would be murder. This is the difference that makes unhooking the violinist and abortion too dissimilar to be analogous.
Finally and most importantly, the act of abortion is the direct killing of an innocent human being, and every civilized society recognizes killing the innocent as wrong. In the case of pregnancy, it is true that the innocent child is connected to and dependent on the mother’s body for survival, but this physical relationship is completely natural. The child is not an intruder who is actively, maliciously, or even intentionally invading and threatening the mother. The child was invited there through the actions of the mother. The womb is the only place we can be if we are to survive the first few months of life. Therefore, killing your own pre-born child (like killing the violinist) is not morally justified, even though she is connected to and needs her mother’s body to survive.
Our right to life is foundational and needs to be protected, even when, especially when we are too young to survive on our own and too weak to defend ourselves against lethal attack.