Timing of administration of antenatal magnesium sulfate and umbilical cord blood magnesium levels in preterm babies.

Background: The optimum timing of administration of magnesium sulfate (MgSO) in relation to delivery is not known. The general consensus is to achieve administration to the mother at least 4 hours prior to preterm delivery.

Objective: To investigate potential predictors of umbilical cord blood magnesium (Mg) concentrations, in particular, timing of antenatal MgSO administration in relation to delivery.

Study Design: A prospective observational study of infants delivered at less than 32 weeks' gestational age. Cord bloods samples were collected at delivery and Mg levels analyzed.

Results: Of the 81 included cases, five received no antenatal MgSO, 65 received a 4 g bolus only, and 11 received a 4 g bolus and 1 g/hour infusion. The median time of bolus administration before delivery was 104 minutes (IQR: 57-215). The mean magnesium level was 0.934 mmol/L in the no antenatal MgSO group, 1.018 mmol/L in the bolus only group, and 1.225 mmol/L in the bolus and infusion group (p < .05). In the bolus only group, the highest mean magnesium concentration (1.091 mmol/L) was achieved with administration 1-2 hours before delivery, but the difference was small and not statistically significant. On multiple regression analysis, lower birthweight Z scores and gestational age were independently associated with higher cord blood Mg levels.

Conclusions: In the bolus only group, the highest mean Mg levels were observed with administration 1-2 hours before delivery, but the findings were not statistically significant. Compared to the rest of the cohort, higher Mg levels were found when a bolus was followed by an infusion. Following a MgSO bolus, some growth restricted extremely preterm babies may have higher Mg levels than would be otherwise expected.