I read that some women get pregnant on the pill because it's not effective enough for their body. How do I know it's effective enough for mine?

Faith. Sometimes in this world, one has to live by faith. Birth control pills work in almost every woman, almost all the time. That's good enough for life. Just like treating a strep throat infection... we know that penicillin works almost every time, but not every single time. When a person with strep throat starts her penicillin, she has faith it will work. There are no 100% guarantees.