A vasectomy can indeed effectively ‘reverse’ itself, but for most men it means nothing.
There is a well documented phenomena called ‘spontaneous re-canalisation’, whereby a fine track opens up between the cut ends of the vas tubing and on occasion a few sperm may get cross. Any appearance of sperm is fleeting and the numbers of sperm tiny, so it doesn’t neccesarily result in a pregnancy.
It is estimated that this occurs in 1 in 4000 men who have had a vasectomy.
One excellent study from Oxford years ago looked at the incidence of positive sperm counts 12 months after otherwise successful vasectomies (ie where the patients were considered all clear) and found 0.6% men had positive sperm counts. This figure was 18 times greater than the reported pregnancy rate following successful vasectomy.
Thus, for most men, the re-appearance of sperm does not matter.
In the real world, any estimate for the incidence has to be an educated guess as most men who have had a vasectomy do not shout the fact from the roof tops wheh the other half falls pregnant. Indeed, by time their sperm count is re-checked to see if he is fertile again the count may have once again reverted to zero! With DNA testing however, it has been possible to establish parentage and show that the man really is the father.
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