The main reason is that if you take the emergency contraceptive pill during the second half of your period, most of you will not experience withdrawal bleeding because you have already ovulated and your ovaries are producing progesterone, and the endometrium is supported by progesterone, so you will not experience withdrawal bleeding, which does not indicate contraceptive failure. If you do not have your period when it is due, you can use your morning urine to take an early pregnancy test, because although the emergency contraceptive pill is effective, the contraceptive rate cannot reach 100%, so the possibility of pregnancy should be ruled out. It is also recommended not to use the emergency contraceptive pill as a regular contraceptive method, because the side effects are still relatively large.