Do inter-uterine devices (IUDs) work by preventing conception or by killing a conceived prenatal? I sought an answer by searching articles from medical journals at scholar.google.com. I found a study, published in the American Journal of Obstetrics and Gynecology (2002), which answers this very question by analyzing a set of past studies in the peer-reviewed medical literature:
 Joseph B. Stanford, MD, MSPH, and Rafael T. Mikolajczyk, MD
Mechanisms of action of intrauterine devices: Update and estimation of postfertilization effects
American Journal of Obstetrics and Gynecology 2002. vol. 187, no. 6, pp. 1699-1708.
[available as a PDF here]
The study considered 3 types of IUD [Wikipedia on the three types] — The copper IUD, the hormonal IUD, and the inert IUD. The inert type does not contain hormones or copper, and is made of plastic and/or steel. Wikipedia says: “No inert IUDs are approved for use by the healthcare authorities in the USA, UK, or Canada.” So this article will mainly focus on the copper and hormonal types.
The study concluded that each of the three different types of IUDs function both as contraceptives and as abortifacients:
“There are many potential mechanisms of action for the intrauterine device (IUD), which vary by type of IUD (inert, copper, or hormonal). This paper reviews the evidence for each potential mechanism of action. On the basis of available data for fertilization rates and clinical pregnancy rates, the relative contribution of mechanisms acting before or after fertilization were quantitatively estimated. These estimates indicate that, although prefertilization effects are more prominent for the copper IUD, both prefertilization and postfertilization mechanisms of action contribute significantly to the effectiveness of all types of intrauterine devices.” [1. abstract, p. 1699]
The “pre-fertilization effects” prevent conception. So each type of IUD works as a contraceptive. However, the “postfertilization mechanisms of action” result in the destruction of a fertilized ovum (i.e. a conceived prenatal). From a pro-life Catholic point of view, any such mechanism of action is abortifacient. The term “clinical pregnancy” refers to conception followed by implantation, so that the pregnancy can be confirmed by a pregnancy test (or by the continuation of the pregnancy, of course).
Note that the authors of the study did not use the terms “abortion” and “abortifacients” (although several studies cited by them did use those terms). Instead the authors used terms such as “postfertilization loss” and “preventing long-term viability of the embryo.” This choice of terminology suggests that the researchers are not biased in favor of the pro-life point of view.
The researchers concluded that “both prefertilization and postfertilization mechanisms of action contribute significantly to the effectiveness of all types of intrauterine devices.” Each type of IUD, therefore, gains a significant portion of its effectiveness in preventing clinical pregnancy from both contraceptive and abortive mechanisms of action. IUDs are a type of abortifacient contraception.
The study authors were also able to put a number on the effectiveness of IUDs as contraceptives and as abortifacients [Table III, p. 1705].
Several figures stand out from these results. The first is an estimate of how often fertilization occurs for a sexually-active woman using each type of IUD. The copper IUD had an estimated fertilization rate per cycle of 4.1 to 8.1%. The hormonal IUD had an estimated fertilization rate per cycle of 4.1 to 14.0%. And the inert IUD had an estimated fertilization rate per cycle of 15.6%. These numbers imply that IUDs are not particularly effective as contraceptives, and that users of the IUD often conceive without realizing it. Note that failure rates for contraception are usually stated per year, but these rates are per cycle. So, by comparison, condoms have a failure rate of about 1 to 2% per cycle with typical use . As contraceptives, IUDs are less effective than condoms.
The fertilization rate per cycle tells us how often two things happen together: the contraceptive action of the IUD fails and conception also occurs. How can the clinical pregnancy rate be so low per year, when the fertilization rate, for women using IUDs, is so high? The answer is found in the “Estimated postfertilization losses (% of all fertilized ova)” [1. Table III]. This figure exceeds 99% for all IUDs and scenarios. In other words, IUDs are not especially effective as contraceptives, but they are very effective as abortifacients. In almost all cases, when fertilization occurs, the conceived prenatal does not survive.
“With regard to the postfertilization effect of the IUD, it is likely that the majority of this effect occurs before the embryo enters the uterus. As discussed, the low recovery of ova from the uterus in IUD users, as well as the lack of hCG rise in more recent studies of IUD users, suggest that the major postfertilization effect is destruction of the early embryo in the fallopian tube, in the same way that the major prefertilization effect is likely to be destruction of sperm and ova.” [1. p. 1705]
The authors opine that the main postfertilization effect of IUDs (i.e. the abortifacient effect) is not failed implantation, but destruction of the conceived prenatal in the fallopian tube — before it reaches the uterus. There is not even an opportunity for successful implantation.
The authors give a range of estimated “postfertilization losses” (i.e. death of the conceived prenatal) per woman per year ranging from 0.19 to 1.04 for the copper IUD and 0.19 to 1.82 for the hormonal IUD. The inert IUD (not common in the U.S.) had higher estimated losses of 0.72 to 1.97 per woman per year. So for 100 women per year, the values would be:
copper IUD: 19 to 104 losses
hormonal IUD: 19 to 182 losses
inert IUD: 72 to 197 losses
For one million women using the IUD for one year, these numbers would range from hundreds of thousands to millions of “postfertilization losses”. And for a ten-year period, the prenatal lives lost could be as high as tens of millions. These “losses” are the lost lives of conceived prenatal human persons, due to a medical device that is widely claimed to be a mere contraceptive. An individual woman using the IUD could lose as many as one or two prenatal children for each year she uses the IUD.
Abortion is one of the most grave, most harmful sins on the face of the earth. And abortifacient contraception is a substantial part of the abortion problem. Surgical abortions kill tens of millions of prenatals per year. But the IUD and other forms of abortifacient contraception certainly kill millions more.
And yet, time and time again, Catholic authors and commentators, especially online, defend abortifacient contraception and propose various theological rationalizations for the deaths of innocent prenatals caused by the deliberate choice to use abortifacient contraception while sexually active. The Catholic Church cannot help bring an end to abortion as long as so many Catholics speak as if they were apologists for abortifacients, rather than apostles for life.
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 Joseph B. Stanford, MD, MSPH, and Rafael T. Mikolajczyk, MD, Mechanisms of action of intrauterine devices: Update and estimation of postfertilization effects; American Journal of Obstetrics and Gynecology. 2002 vol. 187, no. 6, pp. 1699-1708.
 James Trussell, PhD, Contraceptive failure in the United States, Contraception. 2011 May; 83(5): 397–404.