Vitamin D Metabolites in Amniotic Fluid*

To provide further data on vitamin D metabolism in pregnancy, the concentrations of 25-hydroxyvitamin D (25OHD; n = 72), 24,25-dihydroxyvitamin D [24,25-(OH)2D; n = 70], and 1,25-(OH)2D (n = 59) were measured in amniotic fluid by competitive protein-binding radioassays. At term, the mean (+/- SE) concentrations of 25OHD and 24,25-(OH)2D in amniotic fluid (810 +/- 76 and 37.5 +/- 5.4 pg/ml, respectively) were significantly lower (P less than 0.01) than those at 16-18 weeks gestation (1707 +/- 2.67 and 149 +/- 3 pg/ml, respectively). Similarly, the concentrations of 25OHD in pooled amniotic fluid samples, as determined by high pressure liquid chromatography and UV absorbance detection at 254 nm, were 664 +/- 188 pg/ml at term and 1240 +/- 294 pg/ml at midgestation. In comparison, no difference could be found between the mean concentrations of 1,25-(OH)2D at term (4.3 +/- 0.8 pg/ml) and those at midgestation (3.3 +/- 0.4 pg/ml). However, in 14 of 39 amniotic fluid samples obtained at term (35.9%), the concentration of 24,25-(OH)2D was undetectable, while the level of 1,25-(OH)2D was increased. The reciprocal relationship between 24,25-(OH)2D and 1,25-(OH)2D found in more than a third of the amniotic fluid samples at term may be due to a regulatory mechanism responding to the increased fetal demand for calcium in the final stages of pregnancy.