How effective?

im gonna start birth control but im going to make my boyfriend use a condom most of the time bc of std's (im younger so im not having sex with alot of guys, its just a precaution) but if i didnt use a condom and took my pill perfectly, aka every day in the morning without missing, could i still get pregnant if he went raw? ive been told its 99% effective no matter what, even if he comes inside me (hard to believe) but i dont trust it bc i only heard that from one person. i just consider it bc it was my mom and shes 40 and had sex on birth control since she was 15. ps, does anybody know if ALL birth controls cause weight gain?