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.
Ovarian apoplexy
In the ovaries of sexually mature women, follicles grow, and oocyte matures in them, to prepare for potential fertilization. As a menstrual cycle progresses, a dominant follicle begins to stand out, and in the middle of the menstrual cycle it reaches its maximum size of about 20mm. Then the follicle ruptures, releasing a mature egg (ovulation). The location from which the follicle emerges then forms a temporary cyst, called the corpus luteum, which produces hormones to continue the menstrual cycle and mature the uterine lining. This is the normal ovarian cycle.