I hate to tell you, but both times that I got pregnant, I was on the pill. Neither time was I on antibiotics. The first time, I was even told that I couldn't get pregnant at that time because I had an ovarian cyst---oops!! Guess ol' doc was WRONG!!! The second time, I took my pill at the same time every day (I set an alarm to take my pills.) because my now "ex" husband didn't want me to have another child. When he found out I was pregnant, he was less than thrilled.
I did read somewhere that you should change the type of pills at least every 5 years...I don't know if that is still true though...
I wish you the best of luck! Keep in touch if you need someone to talk to!