It may be nearly impossible to prove exactly how hormonal contraception works within the current state of science. Yet some evidence is clear and compelling that the long held understanding is correct, as one blogger explains:
We know from studies such as the 2007 "Changes in measured endometrial thickness predict in vitro fertilization success" that the thickness of a woman's endometrium—the lining of her uterus—determines the likelihood of the successful implantation of an embryo. If an embryo—a living human by all characteristics - does not implant, it is passed out of the uterus in what's popularly called a miscarriage, or a spontaneous abortion. (If you're interested, this study also showed that oral contraceptive use also altered the quality of the endometrium, the markers associated with "endometrial receptivity.")
We've known for quite a while that the use of oral contraceptives has the capacity to thin a woman's endometrium. The 1997 study "The effects of monophasic and triphasic oral contraceptives on ovarian function and endometrial thickness" found that "endometrial thickness in OC users was significantly smaller than in controls", that is, than in women who were not using oral contraceptives.
A 2001 study of one the most popular oral contraceptives on the market, Yasmin, found that ... oral contraceptives have the effect of thinning the endometrium.
Such evidence is compelling enough for many pro-lifers to err on the side of life.
However, the birth control pill's status as a potential abortifacient has not found universal agreement, even within the pro-life community. For example, a paper published more than a decade ago by the American Association of Pro-life Obstetricians and Gynecologists—whose authors describe themselves as "committed to honoring the sanctity of human life from conception"—concludes based on a thorough evaluation of the available literature that there is not "substantive evidence that hormone contraceptives include an abortifacient mechanism of action."
Further inconsistency among pro-lifers is seen in the fact that greater opposition exists among abortion opponents for the morning after pill than for the regular birth control pill, which in most cases is the same medication taken hours later and in varying dosage, but working in the same way—with, presumably, an increased risk of implantation rather than fertilization being interrupted. Presumably, but not evidentially. I have never understood pro-lifers' squeamishness at the morning after pill absent the same concern with the daily pill—except perhaps the latter hits a bit closer to home for so many.
Of course, empirical evidence is not the sole basis for moral decision-making. Take instances of rape, for example. Fr. Paul CB Schenck, Respect Life Director for the Catholic Diocese of Harrisburg and Liaison to the Catholic Medical Association in Central Pennsylvania, explains,
Plan B [the morning after pill] may be used in a morally licit way in case of rape. If a pregnancy test is negative, A Catholic practitioner may administer Plan B within 72 hours to inhibit ovulation, thus preventing pregnancy without risking abortion. All that is necessary is moral certitude, not empirical certitude, that there is no human life in danger. So it is not as rigid as some assume or represent. It is always about life present, or the deliberate and intentional act against life. In the case or rape, the intent of Plan B is not to stop life, but to stop the act of the rapist from reaching its ordered end: impregnation, which would be a form of success of the violent assault. Therefore, a Catholic health plan may include emergency use of Plan B in case of rape, provided it is accompanied by a pregnancy test.
Commentary within the Christian and the pro-life communities debating these issues has cropped up in recent months, including a provocative post at Patheos providing analysis suggesting the likelihood that fewer human zygotes are lost when the birth control pill is used than when it is not. While not scientifically provable (at this point), such a notion could have radical implications for the policies and practices of people with pro-life convictions. More and more, it seems, younger members of these groups in particular are re-examining and re-evaluating previous thinking. This questioning seems to be prompted by an array of interests including rejection of the seeming inconsistencies within the pro-life movement described above, greater awareness of global issues such as poverty and with that greater sense of responsibility of meeting those global needs in real and immediate ways; and general distrust of the old, modernist categories such as "pro-life/prochoice," "liberal/conservative," or "Republican/Democrat."