• Measurements of amino-terminal telopeptides of procollagen 1 (P1NP), a marker of bone formation, in lactating and non-lactating postpartum women both at 6-8 and at 12-14 weeks post-partum, and to compare these values to those of normal controls. [ Time Frame: 4 months ] [ Designated as safety issue: No ]
  

• Measurements of Parathyroid Hormone-related Protein (PTHrP), markers of bone turnover, calcium and vitamin D metabolism [ Time Frame: 4 months ] [ Designated as safety issue: No ]
  

New mothers who are either almost exclusively breast feeding or bottle feeding; normal controls to match those new mothers.

Inclusion Criteria:

• Healthy Caucasian, Hispanic or Asian women between the ages of 21-45
• Group 1: Postpartum (singleton pregnancy) women who are exclusively breastfeeding, defined as 1 or fewer bottles of supplemental formula/day.
• Group 2: Postpartum (singleton pregnancy) women are non-lactating, which is defined as bottle-feeding or having weaned their baby from breastfeeding for at least 4 weeks prior to study.
• Group 3: Controls - Healthy non-pregnant women who are race and age-matched to the breastfeeding women in group one. The may not have been lactating or pregnant within the last year.

Exclusion Criteria:

• Subjects with cardiac, hypertensive, vascular, renal (serum creatinine of > 1.5), pulmonary, endocrine, musculo-skeletal, hepatic, hematologic or malignant or rheumatologic disease will be excluded from the study.
• Smokers and those with a history of significant alcohol or drug abuse are excluded.
• Baseline hypertension (systolic BP > 160 mm/Hg) or hypotension (systolic BP < 90 mm/Hg).
• Subjects taking any chronic medications except stable doses of thyroid hormone, prenatal, vitamin supplements or oral contraceptives.
• Those who have received any investigational drug in past 90 days will be excluded from the study.
• Women who are currently pregnant will be excluded from the study.
• Women who became pregnant by IN Vitro Fertilization IVF or any hormonal manipulation (i.e. fertility drugs such as clomid) are also excluded, as they may have an altered pre-pregnant hormonal state.
• All women will have a urine pregnancy test performed at each of the two study visits and must not be pregnant in order to continue in the study.
• Subjects are not allowed to donate blood between study visits.
• In order to narrow the statistical variations in the study population, African-Americans are excluded because of demonstrated differences in renal excretion of calcium and vitamin D absorption.