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 < 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- (0H)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.