https://en.wikipedia.org/wiki/A_Defense_of_Abortion#The_Violinist
In "A Defense of Abortion", Thomson grants for the sake of argument that the fetus has a right to life, but defends the permissibility of abortion by appeal to a thought experiment:
Thomson takes it that you may now permissibly unplug yourself from the violinist even though this will cause his death: the right to life, Thomson says, does not include the right to use another person's body, and so by unplugging the violinist you do not violate his right to life but merely deprive him of something—the use of your body—to which he has no right. "
f you do allow him to go on using your kidneys, this is a kindness on your part, and not something he can claim from you as his due."
For the same reason, Thomson says, abortion does not violate the fetus's legitimate rights, but merely deprives the fetus of something—the use of the pregnant woman's body and life-support functions—to which it has no right. Thus, by choosing to terminate her pregnancy, a woman does not violate any moral obligation; rather, a woman who carries her pregnancy to term is a 'Good Samaritan' who goes beyond her obligations.