When the basal body temperature suddenly drops in the presence of high temperature, menstruation will occur within 24 hours. Because after ovulation, due to the influence of the corpus luteum, the basal body temperature will rise about 0.3-0.5℃, mostly around 37℃, and this high temperature state should last at least 12 days, if it is lower than 12 days, it can be regarded as luteal insufficiency in clinical practice. If this high temperature state has been lasting for more than 15 days, the possibility of pregnancy is very high. Therefore, before menstruation, the basal body temperature will change, from high temperature to low temperature, and it will drop about 0.3-0.5℃ before menstruation.