The Standard Days Method (SDM) is a method of family planning that assumes ovulation to be close to the midpoint of the menstrual cycle; fertility falls between days 8 and 19; and is most effective for cycle lengths between twenty-six and thirty-two days. The purpose of this study was to evaluate the assumptions of the SDM with a new data set of 714 menstrual cycles produced by 131 women (mean age twenty-nine) who tracked their fertility with an electronic fertility monitor that measured urinary estrogen and luteinizing hormone (LH). The LH peak was used to estimate the day of ovulation (EDO) and the six-day fertile window. Results indicated the majority (80 percent) of menstrual cycles had EDOs within three days of the midpoint of the cycle (86 percent with cycle lengths between twenty-six and thirty-two days). Approximately 22.5 percent (172) of the cycles had fertile window days outside of days 8 to 19, 10.2 percent (78) before, and 12.1 percent (92) after. However, there is a low probability of pregnancy when women experience short cycles and the early days of the fertile window are outside of days 8 through 19. We concluded assumptions of the SDM outside of the fertile window with long cycles could be problematic. However, the SDM is valid for women who have most cycles within the twenty-six to thirty-two day range.