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Sperm live for as long as 90 days, or 12 weeks, so until that much time goes by, there are potentially sperm in semen that could permit conception. So for that reason it is important to have a semen analysis 12 weeks after surgery to confirm that no sperm are present in the semen. Additionally, it is necessary to ejaculate several times so that the ducts are cleared of sperm that remain in them after the procedure.
The semen should be collected at home and brought in to the clinic by the patient within a few hours. The clinic can provide containers for this purpose. The semen is examined under the microscope in the office, so the results will be available during the office visit.
A second semen analysis should be done one month later (4 months after vasectomy). Once the second semen sample is seen to be negative, then the patient may discontinue other forms of birth control.
Some studies have shown that the second sample at 4 months adds little additional information and that the 3 month sample is all that is needed. There is a small but non-zero risk of being fertile that might be caught if the second sample is done, so it is still recommended.