I collect old medical texts in my specialty area, and I just acquired a copy of Dr. Leo Latz's classic The Rhythm. My personal copy is the 6th revision, 1939. A brief handbook, written in a FAQ or Catechism style, it introduced the first scientific method of periodic abstinence for the regulation of births.
I was struck by the following phrase, found on page 67 in my copy - in answer to question 37:
The human mind is not very hospitable to new ideas, especially if they run counter to others that have been harbored for years and have been handed down by uncounted generations...Scientists do not readily surrender a position which they have held for years, which they have defended in the class room and on the lecture platform and in their writings, all the more, when they are under the impression that they have excellent reasons, based upon years of obsevation, for their opinion. They will oppose contrary theories with all the resources of their position and learning.
Also of note is a quote later in the book of an article by a Reverend John O'Brien in the May 1933 issue of Homiletic and Pastoral Review.
What a sobering uplifting thought for decent Catholic parents when they realize that their privilege of cooperating with God in His work of creation is also a responsibility that must be deliberately accepted or declined. As for the other class of Catholics, a large proportion, if not the great majority, are probably practicing birth control (my note - he means here artificial contraception) already, salving their conscience with the plea that the Catholic law as understood by them is morally impossible of observance.(question 66, page 139 of Latz's book). Latz later on notes that in the period from 1921 to 1928 36% of the clientele of birth control clinics (predecessors of Planned Parenthood) in 4 major cities listed their religion as Catholic. He also quotes a statistic of 700,000 abortions (criminal) in 1930, with 15,000 maternal deaths related to the abortion.
Anyhow, it is an interesting book.
Calendar rhythm as a means of estimating the fertile period for the purpose of family planning usually demands about 10 days of abstinence per cycle. It has an overall suprise pregnancy rate of around 20% (whereas unrestricted non-contracepted intercourse has a pregnancy rate of 80%+) over the course of a year. However, in those women who compulsively track their cycles and who have reasonable regular cycles, the use-effectiveness of calendar rhythm approaches that of more modern methods of NFP - but it does demand more days of abstinence.
I really feel like I learn a lot when I can find the historical perspective on things.