This study investigates the time course of change in calcium in the blood after a pregnant subject takes an oral dose of calcium carbonate (commonly marketed as "Tums"). This information is important for trials investigating whether calcium can reduce postpartum hemorrhage, bleeding after delivery.

Inclusion Criteria:

• Pregnant female subjects at the study institution, admitted for labor (spontaneous, augmented, or induced)

Exclusion Criteria:

1. severe range blood pressure (BP >160/>110) within the 48 hours prior to delivery
2. patient age <18 years or >45 years
3. renal dysfunction with a documented serum Cr > 1.0 mg/dL
4. known history of congenital or acquired cardiac disease or history of arrhythmia
5. patient taking digoxin
6. patient currently taking a calcium channel blocker
7. Weight <55kg or >100kg, or
8. receiving magnesium infusion within 24 hours prior to or during cesarean delivery
9. Prior or planned administration of calcium by the obstetric or anesthesiology teams for clinical indications within 24 hours of study enrollment
10. Patient took a calcium supplement in the past 48 hours
11. Patient status is NPO (nothing by mouth) as ordered by the clinical team

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