Pain due to prolonged labour may lead to hyperventilation and resultant respiratory alkalosis. This will cause a decrease in the level of free ionized calcium in the ECF, resulting in hypocalcemia and precipitating carpopedal spasm. Ref: Primer on The Metabolic Bone Diseases and Disorders of Mineral Metabolism Edited By Clifford J. Rosen, 7th Edition, Page 354 ; Nutrition Therapy and Pathophysiology By Marcia Nahikian Nelms, 2007, Page 196 ; Essentials of Medical Pharmacology By KD Tripathi, 5th Edition, Page 479