The safe period is not necessarily after the menstrual period; the safe period is related to each individual’s longest cycle, shortest cycle, and the time of the start of the menstrual period. Safe period is not absolutely safe, sometimes a woman’s ovulation will be early or delayed, when the judgment of safe period will become invalid. Judging the safe period can only reduce the probability of pregnancy and cannot be used as a method of contraception. Safe period is divided into pre-ovulatory safe period and post-ovulatory safe period, from the day of clean menstruation to the first 3 days before the start of the pre-ovulatory period, this date is called the pre-ovulatory safe period, that is, after the menstrual period of this period. This safe period is not reliable, because some women can be affected by changes in the environment and mood swings and other influences to make ovulation earlier, so that the pre-ovulatory safe period will be shortened, and they do not know, at this time, the pre-ovulatory safe period is not very safe. If you have sex without contraception, there is a part of the possibility of accidental conception.