Base Body Temperature (BBT) is the sublingual body temperature of the mouth measured early in the morning after waking up (6-8h of sleep) and before any activity, and is the temperature of the body under the most basic activity. The basal body temperature of women fluctuates periodically with the change of hormone level in the body. (A) Measurement method The mercury thermometer (not electronic thermometer) is shaken to below 35 degrees in advance at night, and measured in the morning after waking up, placed under the tongue for 5 minutes, without talking, moving or eating before measurement. The measured results will be recorded on the basal body temperature chart, and the time of menstruation and the number of sexual intercourse should be recorded at the same time. Any other conditions can be specifically noted, such as cold, fever and lower abdominal pain during the cycle, spotting vaginal bleeding, etc. In women with normal menstrual cycle, BBT is biphasic, i.e. it is 0,3-0,4℃ higher in the second half of the menstrual cycle than in the first half. (II) Clinical application BBT is a simple and easy method that can reflect the cyclic activity of the ovaries. 2-3 cycles of measurement are generally sufficient to infer the ovulation pattern and predict the ovulation time. 2.Predicting the date of ovulation Generally, the day of the lowest body temperature in the middle of menstruation (sometimes the lowest temperature is not reflected) or 1-2 days before the increase of body temperature is the day of ovulation. 3.Guiding conception and contraception The most fertile day is within 4-5 days of the junction of low and high body temperature, before that is the relative safe period, and after that is the absolute safe period. 4.Diagnosis of early pregnancy A high temperature phase lasting for more than 20 days can be diagnosed as early pregnancy. 5.Diagnosis of luteal insufficiency If the high temperature period is shorter than 10 days and the difference between high temperature and low temperature is less than 0.3℃, luteal insufficiency should be considered. Luteal insufficiency is one of the factors that cause infertility. (C) Precautions 1. Basal body temperature cannot confirm the exact time of ovulation, but can only give a possible range of ovulation time, which can be used to guide conception or contraception. 2. Biphasic body temperature does not necessarily mean ovulation. For example, in patients with unruptured follicular luteinization syndrome, the basal body temperature is biphasic, but there is no ovulation. Some studies have also shown that in cycles where ovulation is monitored, the basal body temperature is monophasic, but ovulation is present. Therefore, it is best to use a combination of indicators to determine the presence or absence of ovulation.