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.
Calendar-based contraceptive methods
The postovulatory (luteal) phase has a normal length of 12 to 16 days, and the rhythm method formula assumes all women have luteal phase lengths within this range. However, many women have shorter luteal phases, and a few have longer luteal phases. For these women, the rhythm method formula incorrectly identifies a few fertile days as being in the infertile period.