How effective is "Nuvaring"?

I read the entire leaflet ok, but I am very nervous about it. I have been using the nuvaring for 3 months now. I always count the days to know when I will be ovulating and try not to have during those days or try to have my husband wear a condom. Well this month I counted my ovulation day to be on the 10th we had unprotected on the 9th and 12th. I have 2 beautiful babies and not ready for a 3rd. I am so nervous; my mind is playing tricks on me.