The menstrual cycle lasts about 25 days and ovulation occurs between days 6 and 15 of the menstrual cycle. In women who normally have regular periods, the day of ovulation is about 14 days before the next menstrual period. Ovulation refers to the day of ovulation and the 5 days before and 4 days after the day of ovulation for a total of 10 days. When a woman has a regular menstrual cycle of 25 days, the day of ovulation should be around the 11th day of menstruation. Women who rely solely on their menstrual cycle to calculate ovulation may have errors, and the exact date of ovulation can be improved by ultrasound, temperature measurement, and cervical mucus examination in women. Women’s ovulation is not fixed, women’s ovulation process is affected by the external environment, emotions, medication, health conditions and other factors, can appear ovulation day advance and retreat; and occasionally may not ovulate, so the failure rate of the safe period contraception is higher. When women need to accurately calculate the ovulation period, they can improve the examination under the guidance of professional doctors to determine the relatively accurate date.