Abortion is often put into terms of a right of a woman to do what she wants with her body. The unborn child is seen not as a separate human being, but part of the woman.
While it is true that the child relies upon the mother for his/her life, that child is a separate human being.
St. John Paul II quotes the Declaration on Procured Abortion in his encyclical Evangelium Vitae.
Furthermore, St. John Paul II argues that while evidence for a soul is not scientifically verifiable, the science is clear that everything is there for the development of a human person.
The Church believes that God creates a new human life at the moment of conception, and as a result it is no longer a "part" of the mother's body, but an entirely separate human person.