Define requirements of Calcium during pregnancy period?

The foetus retains about 25-30 g of calcium, over the course of gestation, most of which is deposited in the last trimester when the foetal skeleton is growing rapidly and the teeth are forming. On an average, the foetus draws 250-300 mg Ca per day from maternal blood supply. Additional calcium is believed to be stored in the maternal skeleton as a reserve for lactation. So an intake of additional 500 mg Ca is recommended during both pregnancy and lactation. Extensive adjustments in calcium metabolism occur. Efficiency of calcium absorption is better and urinary losses are   lower in pregnancy compared to the non-pregnant state. Hormonal factors play a role. Human chorionic somatomarnmotropin (produced by placenta) enhances the rate of bone turnover throughout pregnancy. Oestrogen largely from placenta inhibits bone resorption, thus provoking a compensatory release of parathyroid hormone.  PTH maintains the serum calcium level while enhancing intestinal calcium absorption and decreasing its urinary excretion. Along with calcium and phosphorus, adequate vitamin D is required.