The decrease in menstrual flow after taking the emergency contraceptive pill may be related to the following factors: 1. The emergency contraceptive pill is a highly effective progestin, the mechanism of contraception is mainly to inhibit ovulation and change the local endocrine situation of the ovaries, so it may temporarily reduce the amount of menstruation, and most of them will return to normal when they have their period next month. 2. If there is a decrease in menstrual flow, the possibility of pregnancy should be ruled out. The effect of the emergency contraceptive pill is not 100%, some people still have a small amount of vaginal bleeding after pregnancy, mistaken for menstruation, it is recommended to do early pregnancy test or blood HCG, if the result is negative, do not be nervous. 3, it may be because of the emergency contraceptive pill after the withdrawal bleeding, so the next menstruation will show a decrease in menstrual volume, the vast majority of Most of them will return to normal when they have their next period.