how effective is it?

I feel foolish for asking, but me and my boyfriend are only using the pill as protection. We got an STD screening before we had sex together. Fun conversation, right? But necessary. My question is we have sex about 3-5 times a week. He often does ejaculate inside me. What are the chances of pregnancy if I religiously take this and have been for 4 years with almost no (under 5) misses in those four years? I already gave myself a scare when my period was only two days late because my breasts were sore