When referring to the first 7 and last 8 days, it generally refers to the calculation of the safe period. Safe period refers to counting from the first day of menstrual bleeding, the first 7 days and the last 8 days about half a month, need to pay attention to the last 8 days including the days of menstrual bleeding, which is often not emphasized, if you start counting the 8 days from the clean menstruation, it is a mistake. When calculating the safe period, it is generally important to note that it is for people whose menstrual cycles are relatively regular and do not vary much. If the menstrual cycle is relatively short, such as at 24 or 25 days, the last 8 days become less safe the further back you go and the time should be reduced. Women can also download computer software to keep track of when they have had their periods for more than three months and let the software help determine safe periods and ovulation.