Question

After a vasectomy, a man is not considered to be infertile until he has given two samples of his semen up to four months after surgery. this is to make sure that _____.

Answer 1

The most general answer to this question is "to make sure that the vasectomy was successful" 

Specifically, what is measured is whether sperm cells are present in the semen. 

During a vasectomy, the tube that carries the sperm is cut shut, but it could happen that this cut is not perfect or that the sperm can reach the semen through some other vessel. 
To make sure that pregnancy will not happen, the man's seamen has to be tested for the presence of sperm. 

Answer 2

After a vasectomy, a man is not considered to be infertile until he has given two samples of his semen up to four months after surgery. This is to make sure that the vasectomy was successful.