This is a prospective pilot cohort study of post-partum lactating women, post-partum non-lactating women, and matched healthy controls 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 lactating women to non-lactating postpartum women and normal controls. 100 female volunteers between the ages of 21 and 45 years will be recruited to achieve 75 evaluable subjects or 25 in each of the three groups. There are two study visits, one at 6-8 weeks and another at 12-14 weeks postpartum. Normal controls will be seen during the follicular phase of their menstrual cycle and they will be age and race matched to the post-partum women.