Effects of gestational age, maternal diabetes, and intrauterine growth retardation on markers of fetal bone turnover in amniotic fluid.

Little is known about the dynamics of bone formation and bone resorption in utero, particularly the normal changes that occur throughout gestation and in clinical situations that result in low bone mass at birth. The objectives of this study were to describe the effects of gestational age on markers of fetal bone turnover, and to investigate whether the reported low bone mass at birth in small-for-gestational-age (SGA) infants and infants of diabetic mothers (IDMs) was associated with biochemical markers of decreased bone formation or increased bone resorption in utero. Bone formation and resorption were assessed by measurement of carboxyterminal propeptide of type I procollagen (PICP) and cross-linked carboxyterminal telopeptide of type I collagen (ICTP), respectively, in 201 amniotic fluid samples. These markers are by-products of type I collagen formation and degradation, respectively, and have been used in the assessment of bone metabolism ex utero. Both PICP and ICTP concentrations in amniotic fluid were inversely associated with gestational age (P < 0.0001). Amniotic fluid concentrations of PICP increased exponentially in relation to infant birthweight (P = 0.008), and SGA infants had lower amniotic fluid PICP concentrations than controls (P = 0.07). The presence of diabetes in the mother was not associated with alterations in amniotic fluid PICP or ICTP concentrations. Although maturational effects on clearance of bone markers from amniotic fluid cannot be excluded, these data are consistent with a high turnover of bone matrix early in fetal life, and a reduction in bone formation when fetal growth is compromised.