https://www.scientificamerican.com/article/periodic-abstinen...
although we do have this:
> "You really can't extrapolate from this paper," says David Grimes, an obstetrician-gynecologist and vice president of biomedical affairs at the nonprofit public health organization Family Health International. "Naive readers see these results, and they think [STM] is the greatest thing since laptop computers. The researchers on this paper went back and cherry-picked this data from an ongoing study from the past 20 years. They chose the users who were the best users for this method."
and there aren't any references there at all. Besides, the requirement that a couple perfectly adhere to the rules is pretty ... unergonomic.
(caveat: I'm of course not remotely anti-contraception, but it was a fun couple of searches)