Materials and Methods: Forty women aged 18-40, with gestational age of 26-32 weeks and signs of preterm labor, and 40 women in similar age and gestational age with normal pregnancy were included in this study. Case group received betamethasone 12 mg in 2 separate doses with 24-hour interval. MgSO4 was administrated (in dose of 4 mg), and continued by 2 mg/h. Once the tocolytic therapy goal was achieved, the administration of MgSO4 continued for an additional 12-hour.

Results: The mean age, gestational age, gravidity, parity, number of live born children, number of dead children, miscarriage, and molar pregnancy were similar in 2 groups. The mean of serum magnesium level showed a significant difference between 2 groups. In the case group, 27 patients showed positive therapeutic response to MgSO4; the difference between the mean of serum magnesium level in these 27 patients and the other 13 patients who did not respond well was significant. The serum magnesium level < 1.85 mg/dL as a cut-off point had 85% sensitivity and 78% specificity with CI = 0.75-0.97 in predicting response to MgSO4 in the case group.

Conclusion: Serum magnesium level can be used as a predictive tool for preterm labor. It can help in preterm labor in selecting patients who benefit from MgSO4 as a tocolytic agent. Magnesium supplementation may be helpful in patients with decreased serum magnesium level to prevent preterm labor.