The safe period is determined by different methods. According to the law of ovulation, women can avoid having sex during the ovulation period, so that the sperm and egg can not meet, so as to achieve the purpose of safe contraception, this period outside the ovulation is called the safe period. The safe period is generally based on the cyclical physiological changes in the female reproductive system, through the menstrual cycle table projection, basal body temperature measurement and cervical mucus observation to determine the pattern of ovulation. Ovulation is generally estimated by the menstrual cycle table to be about 14 days (12-16 days) before the next menstrual period, so all other times are safe periods except for the 5 days before ovulation and 4 days after ovulation. Adult women ovulation, ovarian luteinizing hormone has a thermogenic effect, stimulate the hypothalamus thermoregulation center, and make the body temperature rise 0.3 ℃ ~ 0.5 ℃, so women ovulation basal body temperature rise, to the menstrual period 2 days before the first day of menstruation or the lth day of menstruation body temperature and decline, so except for the days of rising body temperature, other times are safe period. Before and after ovulation due to changes in ovarian hormone secretion, affecting the amount of cervical mucus secretion, drawing degree and crystallization pattern, according to which you can judge the date of ovulation, other than the time are safe period. However, women’s ovulation is affected by a variety of factors, including emotions, health conditions and external environment, safe period is not very safe, it is recommended to choose in conjunction with their own situation. Safe periods are relative and not 100% safe, so it is advisable to consider them in a holistic manner.