Does the maternal kidney contribute to the increased circulating 1,25-dihydroxyvitamin D concentrations during pregnancy?

The observation that during pregnancy the circulating 1,25-dihydroxyvitamin D, 1,25(OH)2D, concentrations are higher than in the nonpregnant state as well as recent evidence showing that, in vitro, the placenta and/or decidua are sites of 1,25(OH)2D synthesis has led to the general belief that the increased circulating 1,25(OH)2D concentrations originate from the placenta and/or decidua during pregnancy. The observation of a patient with end-stage renal disease who became pregnant after 10 years of chronic hemodialysis treatment has revealed that, despite delivery of a viable infant who had a normal development for gestational age and of normal serum 25-hydroxyvitamin D levels, her serum 1,25(OH)2D3 concentrations were only 10-15 pg/ml following 25 weeks of gestation. These 1,25(OH)2D3 concentrations are far lower than those usually encountered in normal women at the end of the 2nd trimester of pregnancy. It is felt that an important contribution of the placenta and/or of the decidua to the synthesis of the hormone should have led to higher 1,25(OH)2D3 concentrations than those observed in this patient. These observations, along with evidence from the literature, prompted us to reappraise the hypothesis on the origin of the circulating maternal 1,25(OH)2D3 during pregnancy and to postulate that the kidney might be more important than previously thought to the synthesis of the hormone during pregnancy.