How likely?

I've been taking a particular birth control for about a year and took a different kind for a couple months prior to switching due to the way it made me feel. My DF and I want to wait until we're married to have a baby. How likely am I to get pregnant if he cums in me every time we have sex and I'm taking birth control like I should and for so long? I know that birth control is not 100% effective but I would just like to know.