A study in the Feb. 23, 2007, issue of Fertility and Sterility, publicized only this week by NRFC, found that emergency contraceptives comprised of the steroid Levonorgestrel, like Plan B, can cause abortions. (The “delay” the study refers to is length of time following sex the morning-after pill is taken.)
Result(s): With disruption of ovulation alone, the potential effectiveness of levonorgestrel EC ranged from 49% (no delay) to 8% (72-hour delay). With complete inhibition of fertilization before the day of ovulation, the potential effectiveness of levonorgestrel EC ranged from 90% (no delay) to 16% (72-hour delay).
Conclusion(s): The gap between effectiveness of levonorgestrel EC estimated from clinical studies and what can be attributed to disruption of ovulation may be explained by overestimation of actual effectiveness and supplementary mechanisms of action, including postfertilization effects.
The information followed, coincidentally, the May 21 release of a video on YouTube on the historical and current modes of action of the birth control pill – the best explanation I’ve ever seen, and brief, too:
[Video hat tip: Andrew]