This is a prospective observational trial enrolling healthy postpartum women, ages 20 years and older with the intent to breast-feed from the UNC outpatient obstetrical clinics and the UVA outpatient obstetrical clinics. These women were studied at 3-5 visits in the year following their delivery to assess bone density, nutritional intake, exercise, and breast-feeding choices. The number of study visits depended on the timing of return of menses and interest in participating in the follow-up study (timepoints are: immediate postpartum, 3 months postpartum, return of menses, 6 months after return of menses and follow-up visit 6 months after final visit). Blood was taken for analysis of bone-related hormones, bone turnover and calcium homeostasis markers. The study focuses on women with extended lactation (at least 5 months of lactation) whereas women who cease lactation prior to 3 months will be the primary comparison group.