So far as I know, the Church has not definitively ruled on the matter one way or the other, but moral theologians generally conclude that measures may be taken to prevent fertilization of a woman’s egg by sperm after a rape. (It only becomes a “chemical abortion” if a zygote has been created after the meeting of an egg and sperm.) However, if ovulation is known to have occurred and the woman is in her fertile part of her cycle, it is considerably more risky and therefore morally problematic to use those methods of contraception that carry the risk of chemically aborting a child who has been conceived.
