It is important to understand that methods of calculating effectiveness of contraceptives do not work very well for describing natural family planning systems.  Contraceptive methods only work to prevent pregnancy, and they do so by trying to change the reproductive nature of intercourse. As such, any pregnancy is considered a “failure” of the method.   By contrast, couples using a natural family planning method are free to use the method to achieve a pregnancy or avoid a pregnancy – so both achievement of a pregnancy and avoidance of pregnancy can be successful outcomes of the method.