I think it's all just a crap shoot....With my first, I was very surprised because I was 20 and had been having unprotected sex for years with my then boyfriend/fiance. Then wham, one month out of the blue ( we did nothing differently that month) after two years, I was pregnant.
Then with baby #2, he was conceived the second month of trying.
I tried for a baby #3 for two years with my second husband and nada!! I never did get him to get an SA....but you never know...It goes to show you can never tell...
I hope to get pregnant again one day soon!