Calcium-regulating hormones and ions in amniotic fluid.

Concentrations of the hormones and ions involved in calcium homeostasis were analyzed in simultaneous samples of amniotic fluid and maternal blood obtained from normal pregnant women between 14 weeks' gestation and term. Amniotic fluid total calcium, magnesium, and phosphorus levels fell progressively and markedly despite constant or only declining maternal serum levels. Ionized calcium was constant in both amniotic fluid (mean 2.22 mEq/L) and maternal serum (mean 2.33 mEq/L) throughout gestation. Parathyroid hormone levels in amniotic fluid declined after 20 weeks' gestation, despite rising maternal serum levels, consistent with suppressed fetal parathyroid activity secondary to the relative hypercalcemia of late fetal life. Calcitonin levels in both amniotic fluid and maternal serum were unchanged throughout gestation, with amniotic fluid levels being significantly lower than those in maternal serum.