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.
Khul'
When a woman is granted a divorce through khulʿ, she must enter a waiting period known as iddah. During this period, a woman is not allowed to remarry for one menstrual cycle to ensure that she is not pregnant. In case of divorce women has to wait for three menstrual cycles. Most jurists recommend waiting for the three menstrual cycles even for the iddah of a divorce through khulʿ. If a woman has already gone through menopause, she must wait three calendar months. The waiting period for a woman who has irregular periods is dependent on different interpretations. While some jurists believe that a woman should observe the waiting period for three calendar months as well, others believe that a woman should observe the waiting period for nine months. If a woman is pregnant or becomes pregnant during the waiting period, she must observe the waiting period until laying her burden.