The primary aim of the study is to measure bone formation in both lactating and non-lactating post-partum women and compare these to those in healthy non-pregnant controls. The secondary aim is to obtain measurements of Parathyroid Hormone-related Protein (PTHrP), markers of bone resorption, and calcium and vitamin D metabolism in these subjects. The investigators believe that lactating women will have an increase in bone resorption but no increase in bone formation when compared to non-lactating post-partum women and normal controls.