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.
Woman
Menarche, the onset of menstruation, occurs on average at age 12–13. Many cultures have rites of passage to symbolize a girl's coming of age, such as confirmation in some branches of Christianity, bat mitzvah in Judaism, or even just the custom of a special celebration for a certain birthday (generally between 12 and 21), like the quinceañera of Latin America.