TTC after withdrawal

My partner and I have been solely using the withdrawal method for 4 years with no pregnancies. We are now trying to conceive and I'm just wondering how many girls have used the withdrawal method and then successfully gotten pregnant once they started trying?
Reason I ask is because my partner thinks he is sterile, he's not yet been tested but just has a feeling. We've gone 4 years without getting pregnant while not using proper protection. 

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