A vasectomy involves a minor procedure typically done in the office to “snip” the vas deferens, the tube that carries semen to the penis for ejaculation.  A vasectomy is considered a permanent form of birth control. This is done to prevent the release of sperm when a man ejaculates. Without sperm, a man cannot impregnate a woman. Vasectomy is nearly 100 percent effective for preventing pregnancy, but is not full-proof immediately because the sperm needs to clear from your system. For three months following your procedure, an alternative form of birth control should be used (i.e. condoms, the pill, etc.). A simple test called a semen analysis will show when there are no more sperm in your ejaculate. Very rarely, tubes grow back together in which case pregnancy can occur. This happens in about 1 in every 1,000 cases.