Vasectomy is not immediately effective because live sperm remain in the vas deferens until they are ejaculated in the semen. After three months a semen analysis must be taken to check that there are no live sperm in the ejaculate. Once this is established, vasectomy is 99.85-99.9% effective. This means that, on average, of 1000 women whose partners have had a vasectomy, only1 of them will become pregnant at some time in the future.