In the months after a vasectomy, a semen sample will be needed for testing. Three months after the surgery, the semen should have no sperm in it. If there is any sperm in the sample, the man remains fertile and can father children. Testing will be repeated at a later date to see if sperm remains in the semen. Until the semen is completely free of sperm, another form of birth control should be used to prevent pregnancy.
In rare cases, pregnancy can result after a vasectomy and a semen sample that is negative for sperm. In these cases, pregnancy usually results because the two sides of the vas deferens can actually heal together and reform the path out of the body. This is the least likely to happen when the surgeon uses a procedure that cauterizes the cut ends of the vas deferens, leaving scar tissue that makes it much more difficult to form this type of self-repair. This type of cautery can be done with both the traditional and no scalpel (NSV) approach.
In the vast majority of cases, a vasectomy is one of the most reliable forms of birth control. Pregnancy is far more likely to result from not waiting long enough for the active sperm to leave the body following the procedure than from a failure of the surgery. In other words, assume you are 100% fertile until your semen sample is found to have no sperm.
Facts About Vasectomy Safety. National Institutes of Health.
Vasectomy. National Institutes of Health http://www.nlm.nih.gov/medlineplus/ency/article/002995.htm
Vasectomy. Planned Parenthood of Central North Carolina http://www.plannedparenthood.org/centralnc/vasectomy-20843.htm