Objective: Magnesium sulfate (MgSO4) is widely used as a tocolytic agent and its prolonged use has been associated recently with congenital rickets in the offspring. We hypothesized that short-term MgSO4 tocolysis would increase fetal bone resorption and turnover.

Methods: We measured an index of bone formation, carboxyterminal propeptide of type I procollagen (PICP), and an index of bone resorption, cross-linked telopeptide of type I collagen (ICTP) in cord blood of 19 preterm infants born to Mg-treated mothers (mean gestational age:30.4 weeks) and in 19 controls (33.3 weeks).

Results: Mean duration of MgSO4 tocolysis was 37 hours (range: 3-112). Both PICP (3120 ± 1117 vs 2111 ± 715 μg/L; p < 0.01) and ICTP (318 ± 204 vs 89 ± 64 μg/L; p = 0.01) were higher in the Mg group. There was no correlation between duration of maternal Mg treatment and PICP (p = 0.3), whereas ICTP was significantly correlated (p= 0.02). Both indices were inversely correlated with gestational age. When corrected for gestational age, least square means of log PICP and ICTP in Mg-treated infants and controls were 7.90 ± 0.12 vs 7.58 ± 0.08(NS) and 5.46 ± 0.12 vs 5.22 ± 0.08 (NS), respectively.

Conclusions: Short-term tocolysis with MgSO4 does not appear to significantly alter indices of fetal bone turnover.