One of the reasons Humanae Vitae was resisted in 1968 is that women’s reproductive cycles were far less understood than today. The only natural method available then, the much derided rhythm method of spacing children, was a one-size-fits-all template that didn’t take into account the differences between one woman and another or one woman from month to month.

This is why Pope Paul VI, in writing Humanae Vitae, made an appeal to scientists:

Our next appeal is to men of science. These can “considerably advance the welfare of marriage and the family and also peace of conscience, if by pooling their efforts they strive to elucidate more thoroughly the conditions favorable to a proper regulation of births.” (28) It is supremely desirable, and this was also the mind of Pius XII, that medical science should by the study of natural rhythms succeed in determining a sufficiently secure basis for the chaste limitation of…