Emergency contraceptive pills are useful the day after intercourse because they are also effective if taken within 72 hours of intercourse. However, emergency contraceptive pills are not 100% effective and have a 20% chance of failure. If your period is delayed, try to buy a pregnancy test to rule out the possibility of conception. Also, if you especially want to know sooner, you can take a test as early as 12 days after intercourse to rule out conception. You can also go to the hospital for a blood test for human chorionic gonadotropin about eight days after intercourse. As long as the value of human chorionic gonadotropin is below the normal range, you are not pregnant. Birth control pills can cause abnormalities, such as early or delayed menstruation, so observation is recommended.