This is a prospective cohort study of post-partum lactating, non-lactating, and healthy control African-American women who are not currently or have not recently been pregnant. The investigators hope to estimate the measurable differences in bone formation and resorption by comparing blood and urine samples from African-American lactating women to African-American non-lactating postpartum women and normal controls. 100 female volunteers between the ages of 21-45 years will be recruited to achieve 75 evaluable subjects or 25 in each of the three groups. There are two out-patient visits, each 6-8 weeks apart. The visits will take place at the University of Pittsburgh Medical Center (UPMC) - Clinical & Translational Research Center (CTRC) located on 6 North East (6 NE) in Montefiore Hospital. The post-partum subjects will be recruited during their last trimester of pregnancy or at their 6-week post-partum visit. Healthy controls will be recruited and matched to a breast-feeding postpartum woman who has already completed the study. Visit 1 for all postpartum women will take place at 6-8 weeks after delivery and Visit 2 at 14 weeks after delivery. Healthy control subjects will have two study visits at similar intervals to their matched breast-feeding subject (1.5 to 2 months). For all subjects, an informed consent will be obtained at the first visit, before any study procedures are done: including a medical history and measurement of vital signs, an intake history about menstrual cycles, pregnancy, and if applicable, delivery and breast or bottle feeding events. Subjects will complete a dietary calcium intake questionnaire. Blood collections are obtained at each visit for measurements of the following: serum ionized and total calcium, phosphorus, creatinine, albumin, PTH(1-34), PTH(1-84), PTHrP (1-36), 25-hydroxy Vitamin D by High Performance Liquid Chromatography (HPLC), including D2 and D3, 1,25 (OH)2 vitamin D, estrogen status, Luteinizing Hormone (LH), Follicle-stimulating Hormone (FSH), Estradiol, and markers of bone metabolism (e.g. NTX, CTX, P1NP, Osteocalcin (OC), BSAP, Insulin-like growth factor-1 (IGF-1), and selected cytokines of the interleukin-6 (IL-6). Urine samples will be obtained for calcium, creatinine and phosphorus. A urine pregnancy test will be done at each visit for normal controls. Post-partum women will have a urine pregnancy checked at Visit 2 only, to avoid false positives that are likely at Visit 1 due to the recently completed pregnancy. Thyroid Stimulating Hormone (TSH) will be checked at Visit 1 to exclude those women who have significant alterations in thyroid functioning that could interfere with bone metabolism.