Firstly, because late recanalization is very rare (see section “After vasectomy”). Secondly, because when the ducts reconnect, sperm begin to pass immediately. The likelihood that your annual sperm count will be within a few days is slim. Your partner has a much better chance of getting pregnant before you are caught with a positive spermogram. You can have a negative spermogram the day before and a positive one the day after. However, with less than a one in a thousand chance of failure, vasectomy is still much more effective than the birth control pill, condoms or even tubal ligation. In conclusion, if your spermogram 2-3 months after your vasectomy shows that you are sterile, you will no longer need any other method of contraception and you can sleep soundly.