According to the search results, Plan B works by delaying ovulation, which is when the ovary releases a mature egg. If ovulation has already occurred, taking Plan B may not be as effective in preventing pregnancy. However, it is difficult to know exactly when ovulation occurs, so there is no harm in taking Plan B after ovulation. It might still work, and if it doesn’t, it cannot harm you or a fertilized egg. It is important to note that emergency contraception must be used before a pregnancy occurs. Plan B will not end a pregnancy that has already started when a fertilized egg has implanted on the uterus. It is best to use emergency contraception as soon as possible, but within 3 days (72 hours) of having unprotected sex. The sooner you take it, the better it will work. If you are unsure whether you have already ovulated or not, taking Plan B is still an option. However, if you have unprotected sex again after taking Plan B in the same cycle, it might not work. In this case, using a barrier method like a condom is recommended.