Kathleen Parker, one of my favorite columnists, offers a biting summary of the best rebuttals. Read it, enjoy it, follow it up. I've addressed the history of abortion teaching in my Development and Negation treatise thus:
"In the case of abortion, for example, the Church’s teaching has developed toward greater strictness and gravity. Somehow that seems objectionable to many people who nonetheless have no problem with greater moral strictness about warfare, capital punishment, and domestic violence now than in the past; but I shall leave that fact aside as one of more psychological and political than theological interest. To be sure, the Church has always considered abortion immoral; and many early Christian writers condemned it as murder (see, e.g., Didache 2:2 and this list). But that injunction appears to have applied only to women who are unmistakably pregnant, either by their appearance or by the detection of quickening. It was not clear on that account that procuring abortion at any stage of gestation is a form of homicide, which is what the Church teaches now.
St. Thomas Aquinas, for example, held that the process of conception required forty days for boys and eighty for girls before the conceptus was ready for the infusion of the rational soul (Commentary on the Fourth Book of Sentences, d. 31 exp. text.). And that was the common view through the eighteenth century. Abortion prior to said infusion was not held by the Church to be the killing of a human person; it was condemned only as a particularly nasty form of contraception. What changed that, of course, was the development of the modern disciplines of obstetrics, gynecology, and above all genetics.
As soon as it became clear to the Church that even the blastocyst, under normal conditions, was a genetically unique individual member of homo sapiens—twinning being a separate, still controversial case—Pope Pius IX included abortion at any stage of gestation as a form of homicide in his renewed list of offenses incurring excommunication (Apostolicae Sedis ). And so the teaching and discipline remain today. The reasonable-enough assumption has been that whatever is a genetically unique individual member of the species is a human person, not just part of a person such as an organ or a gamete. Disputes about the time or process of ensoulment thus recede into obsolescence. A good defense of that development, for which pro-lifers of varying or no religious affiliation are rightly fond of citing natural science, may be found in Robert George and Patrick Lee, Acorns and Embryos. Granted that science just by itself has nothing to say about moral norms, its considerable relevance to this question is the chief basis for claiming that opposition to legal abortion needs no specifically religious premises. That of course is politically very important.
The change here, then, has not been in the precept that abortion is gravely immoral but in the explanation why: due to the advance of science, the Church now condemns all, or almost all, abortion as murder, not merely abortion after a certain stage of gestation. What’s changed is the understanding of the empirical conditions under which the Fifth Commandment is applicable."Of course there's always the CCC itself for those who, unlike the "ardently Catholic" Speaker, actually believe it.