After the vasectomy, there will be some sperm left in the upper part of the vas deferens tubes. It can take more than 20 ejaculations to clear these sperm from the tubes so, during this time, there is still a risk of pregnancy. Until it has been confirmed that your semen is free of sperm, you should continue to use another form of contraception.
At least eight weeks after the procedure, you will need to produce a sample of semen, which will be tested for sperm. This will also help to identify the rare cases in which the tubes naturally re-join themselves. Once tests have confirmed that your semen is free of sperm, the vasectomy is considered successful and you can stop using additional contraception.
A few men continue to have small numbers of sperm in their system, but these sperm do not move (they are known as non-motile sperm). If you are one of these men, your doctor will discuss your options with you. The chances of making your partner pregnant may be low enough to consider the vasectomy successful, or you may be advised to have further tests or consider other options.