The prerequisite for calculating ovulation based on the time of menstrual flow is a relatively regular menstrual cycle. When the menstrual cycle is regular and constant at around 28-30 days, ovulation occurs around the 14th-16th day of the menstrual cycle, which is 14 days before the next menstrual period. Ovulation is not a day, but a period of time that includes the day of ovulation. If the 29th menstrual period is 28 days, the day of ovulation is around the 13th of the following month, and if the menstrual cycle is 30 days the day of ovulation is around the 15th of the following month, ovulation is between the 10th and 19th of the following month. However, women’s menstrual cycle is not constant. In daily life, various factors such as irregular diet, stressful life, emotional changes, changes in the surrounding environment, and stimulation by alcohol and tobacco may cause changes in women’s menstrual cycle, thus causing the time of ovulation to change or advance or postpone accordingly.