I don’t think anyone would say a baby’s life doesn’t matter. If a woman is raped and becomes pregnant, of course she can choose to carry the fetus to term. I think the issue is that the woman’s autonomy matters, too… that the woman should not be forced to do so… abortion cases are the only ones in which we evoke such a stringent, trumps-all perspective of right-to-life.
Judith Jarvis Thomson wrote an article in the 70s that is pretty famous now. The gist… we do not morally treat the right-to-life as an absolute other cases. Yet in abortion arguments, pro-life positions want a fetus’s right-to-life to be absolute. The fetus is being given more rights than older life, and the woman is expected to be more generous with her body than any other person in any other circumstance. Thomson opens her article with a scenario that is analogous to a rape case: you wake up and find yourself in the hospital. A famous violinist is connected to your kidneys. You are told by another group of people that the violinist is dying, and has such a rare blood type that your kidneys are the only ones that can save him. The violinist is an innocent agent in all of this—he did not ask to be plugged into you. You are told you must remain plugged into the violinist for nine months—that it is your duty. You didn’t ask for it, but it is your duty.
The point of this situation: morally, you can unplug yourself. While it may be very nice for someone to remain attached to the violinist and save the violinist’s life, no one is morally required to do so. People may consider someone selfish for unplugging themselves, but it is someone’s right to do so: autonomy of body.
… And just imagine the implications if this were not the case—if people were expected to sacrifice something as fundamental as the right to their body, should someone else suddenly require it; (Thomson didn’t make quite that point—she points out how a pro-choice position expects women to act above and beyond the ordinary expectations of morality, to make such sacrifices that no one else is required to.) When a woman is raped, she has a developing fetus plugged into her body. She does not owe the fetus anything (any more than the person owes the violinist.) She has the right to unplug herself. She doesn’t have to, but she has the right to.