Here is an interesting information sheet that covers not only different methods of Natural Family Planning but also the most reliable “fertility monitor” machines.
The “problem” with NFP boils down to pinpointing the date of ovulation, especially in women who have double peaks. I’ve wondered for years why there can’t just be a machine that analyzes cervical mucous rather than putting the couple through the “guesswork” of all the “signs” (“Is it sticky?” “Is it cloudy?” and so forth).
Turns out, there are. Of course, they all say they don’t work if you’re breastfeeding, which basically goes to say that NFP doesn’t work at all for women who are breastfeeding. In any case, I’ll be posting individually on my research into these machines.