Fetal bone metabolism in normal and rhesus isoimmunised pregnancies

Objective To construct gestation-specific reference intervals for fetal con centrations of biochemical markers of bone metabolism and assess the effect of rhesus isoimmunisation on these. Methods Fetal blood samples were obtained by cordocentesis from 175 pregnan cies (43 complicated by rhesus isoimmunisation) and assayed for carboxy ter minal pro-peptide of type I pro-collagen (PICP) and cross-linked carboxyter minal telopeptide of type I collagen (ICTP) which directly monitor the rate of bone formation and resorption respectively. Results Both plasma PICP and ICTP were negatively correlated with gestation al age (r = -0.351 and -0.472 for PICP and ICTP, respectively, and P < 0.00 1 for both). In fetuses affected by rhesus isoimmunisation PICP levels were lower (P = 0.030) and more variable (P < 0.001) than expected, compared wi th normal unaffected fetuses. However, no such differences were found in th e ICTP levels. In the fetuses affected by rhesus isoimmunisation there was a significant correlation between haemoglobin concentration and both PICP ( r = 0.504, P = 0.001) and ICTP (r = 0.343, P = 0.030). Conclusions Fetal bone turnover declines from early second trimester to ter m, and may be deranged in fetuses affected by rhesus isoimmunisation.