I guess
not trying seems to be the key. My little brother was conceived after my parents decided they
didn't want any more children (mom was on birth control and antibiotics simultaneously... oops...) Quite a surprise, hehe. Mom was about to get her tubes tied when she went in for the pre-op exam and the doc said, "you're not done yet!" But you'd better believe that she had her tubes knotted off the day after she gave birth to little bro
Stress can play a huge factor, so just relax. Good luck knockin' up your wife!