The mode of action of [emergency contraception, EC] has become he subject of heated debate in North America and in several Latin American and Caribbean countries. The main question is centred on whether or not EC prevents pregnancy by interfering with post-fertilization events. This issue is of importance for many people who consider that a new human life begins at the time that fertilization is completed. Accordingly, interference with post-fertilization events would lead to loss of human life. In spite of a lack of scientific evidence to support a post-fertilization effect, this possibility is used as an argument to turn legal, political and religious constituencies against the availability and use of EC. (Ortiz, et al, 2004)
In order to satisfy my own curiosity and my critics, I've reviewed recent scientific literature related to the question of whether or or not Plan B is abortifacient. I do not wish to appear to be in any way "rooting" for Plan B and/or its supporters. I am not. However, I have been very frustrated by the way that many of my fellow pro-lifers have repeatedly stated confidently that Plan B sometimes acts after fertilization and is therefore abortifacient. I do not believe such confidence is supported by scientific evidence. The goal of this literature review is to present a fair appraisal of the likelihood of postfertilization effects caused by Plan B to the pro-life community.
For the most part my analysis will proceed in chronological order, but I'll begin with a newer article (Croxatto, Ortiz, and Müller, 2001) that provides an brief primer on the relevant reproduction science.
[Hormonal emergency contraception, HEC] prevents pregnancy, in which case it acts after fertilization.The fertile days of the menstrual cycle are the days in which an act of sexual intercourse can give rise to pregnancy.These are the 5 days that precede ovulation and the day of ovulation . Thus, spermatozoa have to wait one to 5 days in the female genital tract before encountering the ovum in most cases. This interval provides an opportunity to interfere with the migration and function of spermatozoa and/or with the process of ovulation. EC pills may prevent the encounter of spermatozoa with the ovum; and, even if the two gametes do come in contact, fertilization may not proceed to completion.
The efficiency of fertilization in human beings is very low in comparison with most mammals. Under ideal circumstances,when intercourse takes place during the most fertile days, the chance that fertilization will take place does not exceed 50%  and it is plausible that even minor alterations in the preceding processes will greatly decrease this probability. It has been shown that HEC pills do interfere with pre-fertilization events . When the pills are taken too late to prevent fertilization there are two possible outcomes: (a) that HEC will not be effective and the method fails; (b) that when a woman uses HEC, she does not know whether she takes the pills before or after ovulation, before or after fertilization. For ethical and logistic reasons, it has not been possible to segregate groups of women who take EC after fertilization so as to assess its effect on the establishment of pregnancy. Hence, there is no direct evidence, either for or against, the hypothesis that EC pills prevent pregnancy by interference with post-fertilization events.
There is no direct evidence against the hypothesis that EC pills prevent pregnancy by interference with post-fertilization events. However, as we shall see, there is much indirect evidence against it. Furthermore, since a negative cannot be proven, the burden of proof is on whomever offers a positive hypothesis. However, in order to satisfy critics who claim that the EC Plan B (a.k.a. DL-Norgestral, levonorgestral, LNG) is abortifacient, research has been conducted in order to establish that pre-fertilization effects can account for the effectiveness of the drug.
Some of the discrete steps of the reproductive process whose theoretical interference by EC could prevent pregnancy are:
- follicle maturation
- the ovulatory process
- sperm migration into and through the fallopian tube, including adhesion of spermatozoa to the epithelium needed to acquire and maintain their fertilizing capacity
- zygote development in the fallopian tube
- zygote transport through the fallopian tube
- preimplantation development within the uterus
- uterine retentiveness of the free laying morula or blastocyst
- endometrial receptivity
- blastocyst signaling, adhesion and invasiveness
- corpus luteum sufficiency and responsiveness to hCG
Figure 1 illustrates the chronology of some of these steps within the normal conceptional cycle of women and the time period within which EC needs to act to interfere with each one of them.
(Croxatto, et al., 2001)