When the menstrual cycle is regular, a woman’s ovulation period is counted from the first day of her next menstrual period, and the countdown of 14 days is the day of ovulation. 5 days before and 4 days after the day of ovulation are called the ovulation period, and menstruation usually occurs about 14 days after the ovulation period. Usually the first 2 weeks of the menstrual cycle are the follicular phase and the second 2 weeks are the luteal phase. The follicles in the follicular phase gradually become larger as time increases. When the follicles increase to more than 18-20mm, they are called mature follicles and may ovulate at any time. After ovulation, the ovaries will form the corpus luteum, which continues to secrete progesterone to maintain the thickness of the endometrium. If a woman does not have a successful pregnancy, the corpus luteum will gradually shrink, progesterone will decline, the endometrium will peel off because there is no progesterone to maintain it, and the woman will have her period. The duration of the follicular phase is usually uncertain and can be long or short. For example, if the days of the follicular phase are 20 days, the woman will ovulate around 20 days and her menstruation will come around 34 days. If a woman ovulates around 10 days, her period will come around 24 days. In addition, if a woman has not had her period around 15 days after ovulation, she needs to go to the hospital for a urine test for HCG to rule out the possibility of pregnancy.