The efficiency of phosphorus absorption increases by 15% during pregnancy. The term infant contains approximately 17 g of phosphorus at birth, mostly in bone and water. The physiological adaptations of the mother that increase calcium retention also help to supply the fetus with more phosphorus. There is no evidence that the EAR needs to increase over that recommended for the nonpregnant women, so the RDA for women aged 14-18 years is 1250 mg/day and for those aged 19-50 years it is 700 mg/day. Based on the need to avoid high serum phosphorus concentrations, and the fact that phosphorus absorption is more efficient in pregnancy, the UL is set at 3500 mg/day, slightly lower than the 4000 mg/day for nonpregnant women.
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