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.
Extended cycle combined hormonal contraceptive
Before the advent of modern contraceptives, reproductive age women spent most of their time either pregnant or nursing. In modern western society women typically have about 450 periods during their lives, as compared to about 160 formerly.