You can prevent pregnancy if you use emergency contraception within the first 72 hours after unprotected sex. Emergency contraception is, in fact, the same hormonal doses used in birth control contraception but in higher doses. A few days after taking emergency contraception, you should start bleeding as you do normally during menstruation. Emergency contraception, in fact, causes early menstruation and prevents implantation of the fertilised egg-cell. If used rarely emergency contraception doesnât causes any serious side effects but if it is used too often, EC can cause irregular vaginal bleeding.