Editor’s Note: The editors present this series (read part 1, part 2, part 3, part 4, part 6) on the recent furor over Plan B as an opportunity for our fellow pro-lifers to slow down, step out of activist mode, and enter into the conversation in a prayerful and thoughtful way.
The Church encourages conversation among faithful scientists and theologians as new science comes to light and as we deepen ethical reflection. Typically such conversations occur in academic settings, but since the recent furor pushed the issue into the open, causing much confusion and scandal, we felt it important to present the latest science and moral reflection in a context of faithful discussion.
We offer this series in a spirit of obedience to the Magisterium, and as an opportunity for faithful Catholics and people of good will to come to a greater understanding of the nuances of the Church’s teaching and the complexities of the science and art of medicine in the difficult situations involving the treatment of women who have been raped. There has been no (and will be no) revision in the Church’s teaching concerning direct abortion or contraceptive sexual acts between spouses. Both are morally illicit without exception.
As with all Truth and Charity Forum articles, opinions belong to the author alone and do not necessarily represent the official position of Human Life International.
The mechanism of action of levonorgestrel emergency contraception (hereafter as LNG EC), sold as Plan B, given to prevent pregnancy after an act of intercourse (or rape) is a politically charged but morally important question. The question of mechanism of action is fundamentally a scientific one.
In the discussion on mechanism of action of LNG EC, there is a lot of confusion concerning terms that are used and how particular studies support or refute a postfertilization effect of LNG EC. This brief article is meant to provide a clear, concise summary of the scientific literature pertaining to the mechanism of action of LNG EC, with special emphasis on the best quality evidence.
Effectiveness of LNG EC to prevent pregnancy depends on when it is given, before or after ovulation.
When given after ovulation, there is good evidence that LNG EC is not effective in preventing pregnancy.1 There is also convincing support that LNG EC given after ovulation (actually after the LH surge which occurs the day before ovulation) neither prevents implantation, nor disrupts an embryo that already implanted.2
Many focus their attention on this scientific literature that indicates LNG EC does not prevent or disrupt implantation when administered after ovulation (or the LH surge) to support the claim that LNG EC is not abortifacient whenever given. The period of time that is concerning, however, is when LNG EC is given before ovulation in the fertile window. Consider the following:
First, when given before ovulation (in the fertile window), there is good evidence that LNG EC is very effective in preventing pregnancy.
A study by Noe et al. in 2010 is the largest (n=337) and most robust to date looking at the effectiveness of LNG EC based on when the drug was administered.3 The day of ovulation was determined by the gold standard of serial ultrasounds to determine follicular rupture (which implies an egg is released). The effectiveness of LNG EC was evaluated based on whether it was given before or after ovulation and whether or not LNG EC was effective was based on the difference between expected and actual pregnancies.
As expected, the effectiveness of LNG EC – given on the day of ovulation or later – was basically non-existent (7 expected and 6 actual pregnancies). In stark contrast, the effectiveness of LNG EC – given in the fertile window (-5 to -1 days before ovulation) – to prevent of pregnancy was complete (13 expected but 0 actual pregnancies).
Second, when given in the fertile window (before ovulation), the predominant mechanism of action is not to prevent ovulation.
Again, the study of Noe et al. addresses the question of mechanism of action with this observation: when LNG EC was given in the fertile window, breakthrough ovulations occurred 62 out of 87 times (71%).4 This led the authors to conclude “FR (follicular rupture or ovulation) occurred in some two-thirds of women taking LNG-EC preovulatory; this suggests that other mechanism than suppression of ovulation prevents pregnancy in these women.” In fact, in a subsequent study (Noe 2011) with a similar robust design, the breakthrough ovulation rate was 80%.5
Note: These studies provides robust evidence that is contrary to the presumption that prevention of ovulation is the predominant mechanism of action for LNG EC given in the fertile window. There is now little scientific support for the presumption that prefertilization effects alone account for the efficacy of LNG EC given in the fertile window. At this point, there should be scientific agreement that the primary mechanism of action of (preovulatory) LNG in the fertile window is POSTOVULATORY, and not primarily to prevent or block ovulation.
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