A woman has about a one percent chance of becoming pregnant within the first few years after her partner undergoes a vasectomy, explains HowStuffWorks. Pregnancy is most likely to occur within three to six months following a vasectomy because sperm can take several months to clear from a man's system.
A vasectomy is a surgical procedure in which the vas deferens, the tube that transports sperm to the urethra from the testes, is severed or sealed, according to WebMD. Men are advised to continue using an alternate form of birth control immediately following the procedure and to have a semen analysis three months later to ensure that no sperm are present in his ejaculate. In rare instances, however, a mass of cells grows that reconnects the severed vas deferens, HowStuffWorks warns. To prevent this, some surgeons remove a section of the vas deferens, rather than just severing it.