There’s still no evidence that Plan B interferes with implantation, and lots of evidence that it doesn’t.
There have been reports that Drs. Mikolajczyk and Stanford (“Levonorgestrel emergency contraception: a joint analysis of effectiveness and mechanism of action.” Fertility and Sterility R. Mikolajczyk, J. Stanford, access to free abstract available, here) have proven that there is an abortifacient effect from the morning after pill (“Emergency Contraception,” EC, or the levonorgestrel-only pill protocol, LNG EC).
In fact, they do not “prove” anything. Mikolajczyk and Stanford derived an equation from actual results from observing oocyte follicle development and ovulation in women. They then used statistical, “virtual” models,to estimate they effects of LNG:
We simulated random samples of 10,000 women presenting for EC for a single cycle each, and we calculated the number of ‘‘expected’’ pregnancies for each simulated cohort of women using both sets of the daily fecundity data. We assumed that women ‘‘presented’’ for EC treatment with equal probability on days –10 to +5 around the day of ovulation
For each of the women within the fecundity window, we used the follicular growth equation to estimate a follicular diameter, which in turn was used to estimate the disruption of ovulation by LNG EC based on the data from the Croxatto study (Table 1). We assumed that effects observed for 12–14 mm, 15–17 mm, and R18 mm groups reported by Croxatto et al. (15) apply to follicles of size up 11.51–14.5 mm, 14.51–17.5 mm, and R17.51 mm, respectively. When LNG EC was administered on a day when follicular size was below 11.5 mm, we assumed that there was zero probability of pregnancy. These conservative assumptions maximized the possible effects of LNG EC to disrupt ovulation and prevent fertilization.With this information, we estimated the ‘‘observed’’ pregnancies within the simulated cohorts.
Durand and Croxatto’s teams studied how LNG EC actually worked in the bodies of real, live women, using biopsies, exams, assays of hormones and serial ultrasounds, as well as animal studies. Mikolajczyk and Stanford actually refer to the Durand study on human women, “On the mechanisms of action of short-term levonorgestrel administration in emergency contraception,” (available free on line, here), but say the evidence from biopsies are “mixed.”
On the contrary, Durand reported on actual labs, ultrasounds and even biopsy samples from actual observations:
The results were highly consistent with the chronological date of sampling because differences longer than 3 days between the histologic diagnosis and the day of the cycle were not observed. A total of 24 out of 33 biopsies from treated cycles with ovulatory features were studied. The rest were excluded because of insufficient tissue sample (one from Group B and D) or because sampling did not correlate with the cycle day (three from Group A and four from Group D). Table 3 summarizes the morphological findings in Groups B, C, and D. No significant changes were observed between treated and control specimens in any of the studied parameters. No significant differences among groups were observed. Of particular importance was the finding that the predecidual changes as evaluated by the
presence of prominent spiral arteries, which are considered
crucial for implantation , were not altered by LNG.
The post ovulatory mechanism is most likely explained by the finding in many studies, including Durands’, which have demonstrated a strong effect on mucus thickness and sperm motility from the Levonorgestrel protocol (LNG EC). Practitioners of Natural Family Planning are familiar with this (natural) effect of (natural) post ovulatory rise in progesterone: when the progesterone levels rise after ovulation, the cervical mucus becomes thick and fertility goes down because the sperm can’t get to the egg for fertilization. The movement of the oocyte down the fallopian tube is slowed also, because the cilia in the tube are affected. The combination of these two phenomenon explains the increased rate of ectopic pregnancy in women who do become pregnant using levonorgestrel only EC and daily pills.
There are definitely problems with EC. It only works when it works for 4 or 5 days before ovulation and, possibly on the day of ovulation. (The oocyte only lives about 24 hours.) This is the first time that contraceptive pills have been made available to men as well as women. For some reason, women don’t use the pill correctly, even when they have them at home. And we have tons of evidence that neither the pregnancy rate nor the abortion rate are affected by making the pill available over the counter. And there’s the increased risk of ectopic pregnancy described above.
However, this “study” appears to be statistics used to argue against observations derived from real life medical experiments in order to prove a pre-conceived position.
Edited 3/27/2012 for formatting problems