Abortion is not 100% effective. I just read a note in the New York State Bar Law Digest about a woman who had an unsuccessful chemical abortion, after her doctor advised her that, because of her fibroids, she wouldn't be able to carry the fetus to term.
Well, he was wrong about the drug dosage necessary to induce an abortion and, as it turned out, he was wrong about her ability to carry the child to term. She gave birth to a living child with serious birth defects.