Easy answer birth control is not 100% effective,the only thing that is 100% effective is not to have sex at all. You see birth control can fail for any number of reasons and in some cases no reason at all. The person could not be taking it properly or taking meds that counteract the BC.
So granted one's chances are reduced while on birth control there is still a risk of pregnancy. Of course one also has to remember that hormonal birth control DOES NOT protect against stds. If you dont know who your partner was with before or while with you then you need to make him use a condom.
DS 13 yr
DS 8 yr (3+ yrs ttc)
TTC #3 since 01/02