Denise Hunnell (an MD and MOJ reader) submitted my question to a service called the National Catholic Bioethics Center (www.ncbcenter.org). Peter Cataldo responded by attaching an article he wrote, entitled Therapeutic Uses of Anovulents, which includes the following helpful discussion:
The theoretical possibility of a possible abortifacient effect from chronic use of therapeutic anovulants for female spouses is not a decisive factor in the moral evaluation of their use under these circumstances. The reason is that at the present time there is no moral certitude that such use causes an abortifacient effect. There is no moral responsibility for effects that are not reasonably foreseen. If an effect does not follow from the nature of a cause or in the majority of cases, but instead follows accidentally and seldom, then it may be considered an unforeseen consequence not morally attributable to the agent (see St. Thomas Aquinas, Summa Theologica, I-II, 20, 5; Summa Contra Gentiles, III, ch. 6, ns. 4 and 7).
Evidence of a causal connection between the regular and long-term use of anovulants and the prevention of implantation of a possible embryo is tenuous. There is both physiological evidence that the changes to the endometrium caused by anovulants are probably not sufficient to cause an abortifacient effect, and theoretical evidence that these changes might prevent implantation. Moreover, there is no way of knowing that an embryo exists who might be affected by the drug. This uncertainty about a possible abortifacient effect disqualifies it as a foreseen bad effect. A possible abortifacient effect would not follow from the nature of the pill because its primary mechanism is the suppression of ovulation, and if an abortifacient effect occurs infrequently it cannot be known with certitude. For these reasons, the therapeutic use of anovulants is not an act that would violate the first condition of the principle of the double effect.
For opposing medical and ethical positions from prolife physicians and a pharmacological expert on the question of the abortifacient effect of oral contraceptives see: Walter L. Larimore and Joseph B. Stanford, “Postfertilization Effects of Oral Contraceptives” and Walter L. Larimore and Randy Alcorn, M.A., “Using the Birth Control Pill Is Ethically Unacceptable” in The Reproduction Revolution: A Christian Appraisal of Sexuality, Reproductive Technologies, and the Family, eds. John F. Kilner, Paige C. Cunningham, and W. David Hager Grand Rapids, MI: Wm. B. Eerdmans Publishing Co., 2000), 179–191; Susan A. Crockett, M.D. et al., “Using Hormone Contraceptives Is a Decision Involving Science, Scripture, and Conscience” in The Reproduction Revolution, Kilner et al., eds, 192–201; Joel E. Goodnough, M.D., “Redux: Is the Oral Contraceptive Pill an Abortifacient?” Ethics & Medicine 17.2 (Summer 2001): 37–51; John Wilks, B., Pharm, M.P.S., Response to Joel Goodnough MD, ‘Redux: Is the Oral Contraceptive Pill an Abortifacient?’ ” Ethics & Medicine 17.2 (Summer 2001): 103– 109; and William F. Colliton, M.D., “Response to Joel Goodnough MD, ‘Redux: Is the Oral Contraceptive Pill an Abortifacient?’ ” Ethics & Medicine 17.2 (Summer 2001): 110–113; and Chris Kahlenborn, Joseph B. Stanford, and Walther L. Larimore,“Postfertilization Effect of Hormonal Emergency Contraception,” The Annals of Pharmacotherapy 36.3 (March 2002), 465–470).