The day after the end of menstruation is the safe period, and there is little chance of pregnancy after intercourse, but relying on the safe period for contraception is not absolutely safe, and contraception by calculating the safe period is only suitable for women whose menstrual cycle is absolutely regular. Because the female menstrual cycle is mainly regulated by the ovary’s cyclic secretion of estrogen and progesterone, and also accompanied by cyclic ovulation, and the endocrine function of the ovary is not absolutely stable, may be due to late night, emotional fluctuations, changes in the living environment, suffering from diseases or the influence of certain medications and other factors and changes, so that the female ovary ovulation time in advance or push back, so as to make the period and the safe period time of the corresponding changes in the period. The time of ovulation and the safe period will change accordingly. If you rely solely on the safe period method of contraception, it is easy to cause unwanted pregnancy due to contraceptive failure.