Menstruation
Menstruation is the most visible phase of the menstrual cycle and its beginning is used as the marker between cycles. The first day of menstrual bleeding is the date used for the last menstrual period (LMP). The typical length of time between the first day of one period and the first day of the next is 21 to 45 days in young women, and 21 to 31 days in adults (an average of 28 days).
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.