Fetal bone metabolism in normal and rhesus isoimmunised pregnancies

Objective To construct gestation-specific reference intervals for fetal concentrations 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 pregnancies (43 complicated by rhesus isoimmunisation) and assayed for carboxy terminal pro-peptide of type I pro-collagen (PICP) and cross-linked carboxyterminal 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 gestational age (r =−0.351 and−0.472 for PICP and ICTP, respectively, and P<0.001 for both). In fetuses affected by rhesus isoimmunisation PICP levels were lower (P=0.030) and more variable (P <0.001) than expected, compared with normal unaffected fetuses. However, no such differences were found in the 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 term, and may be deranged in fetuses affected by rhesus isoimmunisation.