The first day after menstruation most women should belong to the safe period, under normal circumstances women’s menstrual cycle is about 30 days, the duration of menstruation is about 3-5 days, usually not more than 7 days. The day of ovulation should be the 14th day before the next menstrual period, 5 days before and 4 days after the day of ovulation, plus the day of ovulation, a total of 10 days of time belong to the ovulation period, excluding the period of ovulation and menstrual period of the rest of the time belong to the safe period. If a woman’s menstrual period is about 3-5 days, the first day after the menstrual period should be considered as the safe period, but if some women have longer menstrual periods, they may not necessarily be considered as the safe period. Women’s endocrine will be affected by many factors, such as changes in the living environment, overwork, mental stress, chronic insomnia, etc., are prone to cause endocrine changes in the body, leading to ovulation in advance or delayed, and sometimes even accidental ovulation, so the safe period of coitus need to do a good job of contraceptive measures.