The Standard Days Method is a fertility awareness-based method of family planning in which users avoid unprotected intercourse during cycle Days 8 through 19. A prospective multi-center efficacy trial was conducted to test, in a heterogeneous population, the contraceptive efficacy of the Standard Days Method. A total of 478 women, age 18-39 years, in Bolivia, Peru, and the Philippines, with self-reported cycles of 26-32 days, desiring to delay pregnancy at least one year were admitted to the study. A single decrement multi-censoring life table analysis of the data indicate a cumulative probability of pregnancy of 4.75% over 13 cycles of correct use of the method, and a 11.96% probability of pregnancy under typical use. This article describes the study and the results. Results suggest that despite its requirement that couples modify their sexual behavior when the woman is fertile, the Standard Days Method provides significant protection from unplanned pregnancy and is acceptable to couples in a wide range of settings.