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.
Abnormal uterine bleeding
Symptoms include vaginal bleeding that occurs irregular, at abnormal frequency, last excessively long, or is more than normal. Normal frequency of periods is 22 to 38 days. Variation in the length of time between cycles is typically less than 21 days. Bleeding typically last less than 9 days and blood loss is less than 80 mL. Excessive blood loss may also be defined as that which negatively affects a person's quality of life. Bleeding more than six month after menopause is also a concern.