Liberals and conservatives sometimes spar over the definition of pregnancy. Some liberals define the term as meaning the period from implantation of an embryo in a mother’s womb forward. Conservatives often define it as beginning at the point of conception. Quite a lot can seem to depend on the definition, since it can seem natural to think that a contraceptive, for example, works by preventing pregnancy, and an abortion by disrupting it. Thus, if pregnancy is not initiated until implantation, and an abortion disrupts pregnancy, then drugs that prevent implantation would be considered contraceptive, and not abortifacient. Conservatives rightly resist this claim, and do so by contesting the meaning of pregnancy.

But a better strategy might be to accept the liberal definition of pregnancy, but reject the conclusions that purportedly follow from it. On three issues—contraception, abortion, and embryo-adoption—I’ll argue that the liberal definition of pregnancy can actually help clarify what sound morality demands.

There are, after all, plausible reasons to think the liberal definition sound. Pregnancy signifies a relationship between the developing human being and his or her mother, but that relationship is not simply the relationship of being the mother of an embryonic child. Mothers of embryos conceived through in-vitro fertilization (IVF), for example, are not pregnant while their children are in Petri dishes. Nor does it seem to be enough simply for an embryo to exist within some part of a woman for the woman to be pregnant. We are inclined to think that in pregnancy the embryo or fetus is connected to the mother in a special way. So, plausibly, a mother becomes pregnant, as liberals think, at the time that the embryo implants in the mother’s womb. At that time, in addition to being a mother of a child, the woman also stands in a uniquely intimate biological relationship of nurturing and sustenance to the developing child, a relationship it makes sense to call “pregnancy.”

But why do liberal proponents of this definition think that it has consequences for understanding the difference between contraception and abortion? The answer, it seems, lies primarily in their misunderstanding of the nature of contraception. For contraception is not a practice whose purpose is the prevention of pregnancy, but a practice whose purpose is the prevention of the conception of a new human being. Consider: someone who prevents an embryonic human being from being implanted in a woman after IVF is not reasonably thought to be contracepting. Someone contracepts only if they intend to prevent a human being from coming into existence—they act contra-conception.

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So the questions surrounding whether the so-called “week-after pill” Ella—or the contraceptive pill, or an IUD—operates only as a contraceptive really has very little to do with pregnancy. The real question is whether they work exclusively by preventing possible human beings from coming into existence, or whether they ever work by making it impossible for already existing human beings to continue to exist. If they do the latter, they are not exclusively contraceptive.

What about abortion: is it not a disruption of pregnancy? Interfering with the life of a not yet implanted embryo usually takes the life of that embryo, yet it does not, if we accept the liberal definition of pregnancy, terminate a pregnancy. Does this make it difficult to say that drugs or devices that take the life of a pre-implantation embryo are abortifacient?  One approach to the question of what abortion is might say yes.

This approach identifies abortion, or direct abortion, precisely as the intentional ending of pregnancy. But even apart from the question of how pregnancy is to be defined, this is a bad definition of abortion. Ending a pregnancy seems neither necessary nor sufficient for a procedure to be an abortion. Not necessary, because an embryo or fetus could be removed from the mother, thus ending the pregnancy, precisely to save the child’s life, if, for example, the mother was incapable of sustaining the child in the womb. Not sufficient because an embryo or fetus could be aborted without the mother’s pregnancy ending. This, sadly, is what happens when mothers undergo “selective reduction” of embryos when they are carrying multiple children. These mothers abort, but remain pregnant.

It seems more plausible to think that abortion causes the death of an unborn child, and that a “direct” abortion is an intentional killing of the unborn child. Not only does such a definition avoid the problem cases just mentioned, it draws attention to what is wrong with abortion in a way that the “ending of a pregnancy” definition does not. For while ending a pregnancy is, just as such, a serious matter—under most circumstances, mothers surely owe it to their unborn children to provide them with a uterine home until birth—the wrong of direct abortion is, more specifically, the wrong of intentionally killing one’s unborn child, not the wrong of expelling it from the womb.

Moreover, the distinction allows one to acknowledge that there might be cases in which only the ending of pregnancy was intended, and not the death of the child. Would this justify ending a pregnancy if the intention was not to kill? In almost every imaginable case, no: For to accept the death of one’s own child as a side effect of ending one’s pregnancy is unjust in every circumstance except when otherwise both the mother and child will certainly die. So while direct abortions are always wrong on this account, indirect abortions—the kind that are a result of ending pregnancy—are also wrong in the overwhelming majority of cases. Moreover, it would seem reasonable to adopt, as a convention, the practice of referring to all procedures that take the life of an unborn human being, whether intentionally, or indirectly but unjustly, simply as “abortions”: it would then be fair to say that all abortions are morally impermissible.

And yet, some problem cases will, arguably, be settled in a different way. Consider the use of methotrexate when used to address a tubal ectopic pregnancy (methotrexate is also used for “elective” abortions). Given where the embryo has implanted, if nothing is done both mother and child could die. Methotrexate works by inhibiting the growth of the trophoblastic tissue that connects child to mother. It also inevitably results in the death of the embryo. Its use seems to fall into the category of “intentionally ending pregnancy,” yet not into the category of “intentionally killing an unborn child.” Thus, if a mother’s life is truly at risk, and there is no possible way to save the child, then deliberately ending a tubal pregnancy by use of methotrexate might not be an instance of “direct abortion,” i.e., not an instance of intentional killing of an unborn child, and might, in fact, be an instance of rightly accepting death as a side effect. But this possibility is likely to be overlooked if abortion is equated with the intentional ending of a pregnancy.

Consider a third case: the question of embryo adoption and rescue, a vexed issue among many conservative bioethicists. Sound moralists acknowledge that one should never separate the unitive and the procreative aspects of the marital act; in other words, marital sexual acts should be open to new life; and new life should not be created outside the performance of marital sexual acts. But neither, the philosopher Mary Geach has argued, should the marital act be imitated in respect to either function. And so the question arises whether, when a woman becomes pregnant through the implanting the biological child of another, the allowing of the embryo into her womb imitates a function specific to the marital act, for, in Geach’s words, in both the marital act, and in embryo transfer, the woman performs an act “an act of admission whereby she allows an intromission of impregnating kind.”

Several philosophers have argued, however, that it is the generative significance of conjugal intercourse—the capacity of sexual union to result in a new human being—that is essential to its unitive significance—the capacity of the marital act to make the spouses “one flesh.” It is not, in other words, the “capacity to make pregnant” that makes man and woman one flesh but rather the capacity to generate new life.

In consequence, the transfer of a human embryo to the womb of a woman who is not that child’s biological mother, whether to rescue the child from the absurd fate of cryo-preservation, or as the first step of a process of adoption, does not imitate the marital act in respect of any of its proper functions. The liberal definition of pregnancy helps us see this because it makes clear that generating a child, and being pregnant with a child, are two different realities.

Yet accepting this definition does not result, I have shown, in the false claim that a process that prevents implantation of an embryo is thereby “merely” contraceptive. And this definition can help us see that there are some procedures which end a pregnancy but are not acts which intentionally take the life of an unborn child. Acts of the latter sort are intrinsically wrong and always morally impermissible, but some acts of the former sort may be morally permissible when otherwise both the mother and child will certainly die.

Editor’s note: The original version of this essay incorrectly stated the views of Mary Geach. We apologize for the error.