For women of reproductive age with regular menstrual cycles the menstrual cycle ranges from 21 to 35 days, with an average of 28 to 30 days. Ovulation is usually about 14 days before the next menstrual period or about 16 days after menstruation. If the period is 3 to 7 days, then ovulation occurs about 9 to 13 days after the menstrual period clears. Having intercourse during ovulation can significantly increase the chances of pregnancy. In contrast, if no pregnancy is planned, it is important to use effective contraception during this period to avoid unwanted pregnancies. However, for women with irregular menstrual cycles who do not have a fixed ovulation period, ovulation can be detected by ovulation test paper or pelvic ultrasound to monitor the size of the follicles in order to clarify the ovulation period. When the ovulation test changes from strong positive to negative, and when the follicle reaches about 2 cm in diameter, it indicates that ovulation is imminent. Ovulation can also be affected by external conditions, such as staying up late or environmental changes or even emotional changes, which can cause ovulation to occur earlier or later.