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      To the Editors:

      Mishell misquotes me in each of his first two sentences. In my article, I did not speculate as to which of the two general mechanisms of action of IUDs, prevention or fertilization versus prevention of implantation, was the “main” mechanism. This question has not been directly or conclusively addressed in the literature. What I did conclude is that inhibition of implantation remains a major mechanism of action of IUDs. Whether the relative proportion of importance of the two mechanisms is 60:40, 50:50, 40:60 or some other proportion, and whether that proportion changes with time, is unknown.
      As have others, Mishell has chosen to ignore the evidence I present. The disproportionate protection against intrauterine versus ectopic pregnancy noted in several clinical trails of IUDs, the studies of ectopic pregnancy among IUD users, and the virtual 100% effectiveness of the IUD when used as an emergency contraceptive compel the conclusions I reached.
      Mishell misses the point. In two of his publications,
      • Mishell DR
      Contraception, sterilization, and pregnancy termination.
      • Mishell DR
      Intrauterine devices.
      when addressing the mechanism of action of IUDs, he states it to be exclusively spermicidal with no mention of a postfertilization mechanism. I believe these publications to be erroneous on this point and therefore might mislead both the providers and recipients of IUDs.
      I am more than willing to concede to Mishell that the spermicidal effect of IUDs might be the “main” mechanism of action. (All this requires is that its proportion of action exceeds 50%.) However, the available evidence requires Mishell to concede a major postfertilization mechanism as well and adjust his future publications to reflect this change. How about it, sir?

      References

        • Mishell DR
        Contraception, sterilization, and pregnancy termination.
        in: Comprehensive gynecology. Mosby, St. Louis1987
        • Mishell DR
        Intrauterine devices.
        Fertil Control Rev. 1992; 3: 3-12