Each of your testicles makes sperm (male reproductive cells). Sperm travel from the testicles to the penis through one of two tubes (vas deferens). On the way, sperm mix with other fluids to form semen, which leaves the body during ejaculation. During a vasectomy, each vas deferens is cut. This prevents sperm from leaving the body. This means that after a vasectomy, your sperm won't be able to fertilize an egg. A vasectomy can often be reversed, restoring the flow of sperm out of the body.
Some pregnancies happen in the first few months after a reversal. But it may takes at least 4 months. Pregnancy rates depend on how soon the reversal is done after a vasectomy. The sooner the reversal, the faster the sperm return and the higher the pregnancy rate.