I somehow get the feeling that if you do find out she is pregnant that will be the deal breaker for you.
And if it is a deal breaker, you will still need to come to terms that your son will have a half sibling.
Somewhat unrelated story... When my wife gave birth via C section to our 2nd (daughter), she had her tubes tied at the same time (the factory was closed). She then insisted that I have myself fixed also. I went to see my doctor and told him that she wanted me fixed so that if we ever got divorced that I could never father another child. My doctor told me that my wife was very selfish and that he refused to let me have the operation. What a relief for me!
