The morning after pill works by preventing, or delaying, ovulation. This means that when the sperm enter the fallopian tubes, there is no egg there to fertilise so no pregnancy can occur. By taking emergency contraception as soon as possible after unprotected sex you reduce your chances of ovulating. If ovulation takes place between unprotected sex and taking the morning after pill, the pill will be ineffective in preventing pregnancy.
Remember that no contraceptive is 100% effective. The effectiveness of the morning after pill depends on when you ovulate and the time between having unprotected sex and taking emergency contraception.
If you have taken emergency contraception but your next period doesn’t arrive, or is over 7 days late, or is unusually light or unusually heavy or if you experience symptoms such as abdominal pain, nausea, vomiting or breast pain or if you have any doubt about being pregnant, then you should take a pregnancy test.