The reason for taking Eusebio for 21 days is mainly because of two contraceptive mechanisms: First, Eusebio is taken during the follicular phase to inhibit the ovulation function of the ovaries, so that without ovulation, the sperm cannot unite with the egg even if it enters the fallopian tube and cannot form a fertilized egg, so the purpose of contraception is achieved. Second, after taking eusebio, especially in the follicular phase, to gradually make the endometrium thinner, after up to 21 days of taking enough to make the endometrium thinner, thin to make the fertilized egg can not be bedded in order to achieve the purpose of contraception, so eusebio to take 21 days in a row, will play a very good contraceptive effect.