In the early 1990s, researchers at Georgetown University, Pennsylvania State University, and the University of Utah proposed a five-year longitudinal study of female reproductive aging that would include the collection of hormonal, menstrual cycle, and health data from a group of women in order to advance the current understanding of the transition through menopause. The women selected for the BIMORA project were a subset of women belonging to the Tremin Research Program on Women's Health (TREMIN), a longitudinal, prospective study of menstrual cycles and female reproductive health that was begun in the 1930s by Dr. Alan Treloar at the University of Minnesota. As part of the TREMIN study, women recorded their menstrual cycles on calendar cards and were also asked to fill out annual and later biannual health surveys. The first cohort of women was recruited in the 1930s when many of them were attending the University of Minnesota. Some of their daughters, along with additional women, were recruited in the 1960s as part of a second cohort. Recruitment continued after the second cohort, and a total of 156 TREMIN women participated in the five-year BIMORA project. At the beginning of the study, they ranged in age from 25 to 58 years of age and many were from the second TREMIN cohort. Women could not be using exogenous hormones and had to have at least one intact ovary. The participating women had TREMIN data going back as far as the early 1960s, and they continued sending menstrual bleeding and health data to TREMIN during the BIMORA project. In addition, from January 15 to July 14 in each of the five years of the BIMORA project, participants collected daily urine specimens and made a daily record of medication use, health conditions, and menstrual bleeding. These data were analyzed in the BIMORA laboratory. The urine specimens were assayed for urinary conjugates of estrogen, progesterone, LH, and FSH. The TREMIN data and laboratory data were then merged into a single dataset.