There is a literature among Catholic scholars that asserts that, contrary to widespread popular opinion, the empirical predictions of the Humanae Vitae (1968) have been vindicated. In short: Everything you think you know about the 'sexual revolution' is wrong. This article is a good example.
In this article, note also the (to me) strange and I think suspect link asserted between the sexual revolution, homosexuality, and the sexual abuse scandals of the Catholic Church.
I came across this citation while reading a William Saletan column, which, if I can decipher the jargon, indicates that ovulation may still occur during the postpartum breast-feeding stage. Does this suggest that, during this stage, a woman may have a fertilized egg that does not implant due to breast-feeding? This would place breast-feeding as an abortifacient practice in line with other methods of contraceptive unacceptable to pro-lifers.
Saletan's earlier column does a nice job of capturing the scientific uncertainty over what happens with eggs and implantation with emergency contraception (like Plan B).
Note how the most important finding (or non-finding) was buried in this article: While contraceptive use was affected by price shifts, there was no statistically significant change in the number of accidental pregnancies.
This study, from Slate, has more nuanced reporting on the question of accidental pregnancies, suggesting that accidental pregnancies did go up for a subset of women in the study.