Amniotic fluid osteocalcin concentrations in pregnancies uncomplicated by fetal bone disease

Osteocalcin is the major noncollagenous bone protein. Serum osteocalcin concentrations increase in conditions when bone turnover is increased, reflecting enhanced osteoblastic activity. The presence of osteocalcin in amniotic fluid (AF) has not been reported. We undertook this study to determine if osteocalcin is present in AF and to establish standards for AF osteocalcin concentrations, to be used in pregnancies at risk for diseases with increased fetal bone turnover. One hundred and thirty-one samples of AF from 15 to 43 weeks gestation were analyzed for osteocalcin by a radioimmunoassay detecting greater than or equal to 0.1 ng/ml, with a coefficient of variation of 6%. Osteocalcin measured in AF had the same gel-filtration characteristics as serum osteocalcin and the standard curves for serum and AF osteocalcin were parallel. Indications for amniocentesis were genetic study (n = 15) and fetal lung maturation (n = 116). None of the pregnancies studied produced an infant with bone mineral disorder. In 67% of the AF samples, osteocalcin was undetected; in 95% of the samples the concentration was less than 2.4 ng/ml. Concentrations of AF osteocalcin did not correlate with gestational age or birthweight, and were lower than the reported concentrations in cord serum or maternal serum. Thus normal amniotic fluid osteocalcin concentrations are less than 2.4 ng/ml; we speculate that its elevation in fetal bone disorders may provide a genetic counselling tool.