I haven't used contraception, why haven't I got pregnant?

Every woman I know who has had a baby has fallen pregnant due to an "accident" (missed taking the pill, was on antibiotics, broken condom etc). Lately I have been having a lot of unprotected and have been very neglectful with my contraception - missing pills regularly, taking antibiotics at the same time (which renders the pill ineffective) etc. Should I be worried that I have never had a pregnancy scare?