they say the odds are like 0. 1%. personally i lack faith in that since my oldest niece and my youngest son were conceived on birth control without missing any pills. but it is unlikely. but a condom is still important. birth control does not protect from stds. so unless you know for a fact that your partner is free of all stds its always best to use a condom.
what are the chances of becoming pregnant while using a contraceptive and no condom?