If the patient has had regular periods in the past, the first 7 days of the period are usually a safe period. However, the safe period is just a lower chance of pregnancy, but not an absolute prevention of pregnancy. The menstrual cycle is influenced by many factors and can be early or wrong. With a disrupted menstrual cycle, there is uncertainty about the ovulation period. The safety period is divided into a pre-ovulatory safety period and a post-ovulatory safety period. The post-ovulatory safety period is from the first day after ovulation to the day before the next menstrual period, which means that the week before the menstrual period is the post-ovulatory safety period. The ovaries rarely ovulate twice in a menstrual cycle, i.e. the period between ovulation and the next menstrual period usually does not result in a second ovulation, so the post-ovulatory safety period is relatively safe, but it is not absolutely impossible to get pregnant. Because the safe period is related to each woman’s longest cycle, shortest cycle, and the time when her period starts, the safe period is not absolutely safe. Therefore, if a woman does not want to get pregnant, she must take good contraceptive measures during intercourse to avoid unwanted pregnancy.