The terms “rhythm” and “calendar” methods refer to methods developed in the 1930s and 1940s based on observations of the menstrual cycle. While these were important advances in the understanding of women’s health at their time, current natural methods of family planning are based on a much more sophisticated understanding of the cycle. The Creighton Model FertilityCare System is based on over 40 years of scientific research and is effective for women who are breastfeeding, premenopausal, or have irregular cycles.