Menstrual synchrony
Strassmann argued menstrual synchrony defined as menstruation overlap should be quite common. For example, the expected difference by chance between two women with 28-day cycles—which is approximately the average length of menstrual cycles of women at the age —is 7 days. Considering that the mean duration of menses is 5 days and the range is 2 to 7 days, the probability of menstruation overlap by chance should be high.
Natural Cycles
The most recent published clinical study showed that natural cycles, when used correctly, is as effective in preventing pregnancies as the contraceptive pill.
The study analysed 22,785 women through a total of 224,563 menstrual cycles across a year, to calculate the app’s Pearl Index – the rate used to measure a contraceptive effectiveness. It found that if used perfectly – using protection such as condoms on red days – effectiveness is 99 percent. Typical use, where people become pregnant for all possible reasons, leads to 93 percent effectiveness. This is well above other Fertility awareness based methods that rate at around 75 percent and even the contraceptive pill, which rates at 91 percent. Further studies include the “Identification and prediction of the fertile window using Natural Cycles" and “Fertility awareness-based mobile application for contraception”. For the app to remain effective, women need to follow the app's instructions correctly, and it does not protect its users from sexually transmitted diseases.