The success rate of vasectomy is more than 99% when we speak about preventing pregnancy. However, a man will not be sterile instantly after the vasectomy surgery. If pregnancy occurs after vasectomy, it is generally because of having intercourse within the first few weeks after the surgery. This is because the man still has sperm in his semen. If you had a vasectomy, you will have to produce a minimum of 20 ejaculations to make sure sperm is no longer present in the semen. The vasectomy will be effective in preventing pregnancy only after the sperm count reaches zero.
You will have to use any form of birth control option with your partner before a vasectomy becomes effective in order to make sure your partner does not get pregnant. After several weeks post the surgery, you can have your semen tested and it is safe to have intercourse without contraception after the results show there is no sperm in the semen.
Recanalization can also be a reason for pregnancy after a vasectomy, but such cases are very rare. During the vasectomy surgery, the surgeon cuts and seals the vas deferentia in order to block sperm from coming out during ejaculation. Recanalization is a rare occurrence where the tubes rejoin and the sperms are unblocked from entering into the semen and can lead to pregnancy.
As vasectomy is 99% effective in preventing pregnancy, you must carefully weigh the options before undergoing the surgery. Though you have the vasectomy reversal option left, discussions are still going on to prove how effective the procedure is. Before undergoing the surgery you must be sure that the probability of having children after a vasectomy reversal is not 100%. Also, a reversal may cost thousands of dollars and success depends on how long ago the vasectomy was originally performed and whether antibodies to sperm have been formed.