As with any vasectomy, the rate of success is greater than 99%. After a vasectomy, sterilization is not instantaneous. Sperm cells which have already traveled up the vas before the operation will be stored at the upper end of the vas near the prostate gland. You will not be considered sterile until the stored sperm is passed. The staff will help you arrange to have your seminal fluid checked after 25 - 30 ejaculations. This is generally done after three months. You should continue to use another form of contraception until two consecutive specimens are found to be sperm free. At this point, it is safe to assume that you are sterile.