I am reminded of the old joke, “What do you call people who use natural birth control? Answer: Parents.”
I do not think I ever had a regular cycle unless I was on the pill. So keeping track of my cycle would never have worked. Many things can affect a woman’s cycle as well, even women who are usually regular. Since sperm can live in the body for days, a pregnancy can easily happen if ovulation is off from the calculated day.
Natural family planning does not exclusively rely on number of days in one’s cycles. The sympto-thermal method also relies on temperature, vaginal moisture, and detecting where your cervix is and it’s texture (high/low, open, not). Even people who are in menopause can figure out what’s going on, usually.
Another thing is natural family planning is most helpful to s...p...a...c...e... children— not eliminate their possibility entirely. When combined with ecological breast feeding, which delays resumption of fertility after childbirth, many people can experience success in having a family they can sanely and affordably raise. With no hormonal birth control needed.