cervical mucus, body temperature when we are in resting mode. when we wake up the first thing we do is take the temperature. it was noticed by the end of the 1800s that women's body temperature will rise 5.4 fahrenheit after ovulation occurred. -- .4 fahrenheit after ovulation occurred. >> they found other signs that made sense. when pope paul wrote humanae ta known, right? the practice of the use was still being developed? >> yeah, i read there were some studies released around the time that humanae vitae was released. the temperature only methods were proving to be 99% effective. i think it was later in the 70s that the rules for the thermal method using the three signs crosschecking ovulation happening, i think those rules were developed in the 70s. >> it was in 1971, when it was created. a couple got inspired by humanae vitae. they saw a need to teach couples about the method and formalize it. make it easier to understand so they can learn. it was in 1971, three years after humanae vitae , when the couple was starting to science and the pope