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.
Estradiol valerate/hydroxyprogesterone caproate
EV/OHPC as a combined injectable contraceptive has a relatively short duration and is associated with a high incidence of menstrual irregularity, for instance polymenorrhea (short and hence fast cycles). This may be unacceptable to many women. Twice-monthly administration of half doses has not been found to improve breakthrough bleeding, though cycle length increased to 20 to 24 days.