The only way to know if the morning after pill has been effective at preventing pregnancy is if your next period arrives when it should. The morning after pill works by delaying ovulation so that you don’t release an egg for remaining sperm in your system to fertilize. Since sperm can live in your body for five days, you should take the morning after pill when you have had unprotected sex, even if you are currently not ovulating, to effectively prevent pregnancy.
When taken within the 24 hours after you have had unprotected sex, your chance of getting pregnant is minimal. If your next period is more than seven days late, you should take a pregnancy test even if you have no other symptoms. Taking a test immediately after taking the morning after pill will not be effective as there will not be enough human chorionic gonadotropin (HCG) in the body yet if a pregnancy occurred. Once pregnancy occurs, the HCG in your body will increase until it peaks. This is what a pregnancy test detects and it will need to be a certain level to register a positive. You can take a more sensitive pregnancy test on the first day of your missed period, but if it is negative and your period has not arrived a week later, you should retest.