Calcium supplementation during pregnancy may reduce preterm delivery in high-risk populations.

Results are presented of a randomized, double-blinded controlled clinical trial of calcium supplementation (2.0 gm of elemental calcium as calcium carbonate) and a placebo. All participants were 17 years of age or less and clinically healthy. Patients were enrolled by the twenty third week of gestation. The mean duration of calcium supplementation or placebo was approximately 14 weeks. Treatment consisted of 2.8 (+/- 1.5) tablets per day in the placebo group (N = 95) and 3.0 (+/- 1.4) tablets per day in the calcium group (N = 94). Dietary calcium intake was similar in both groups at about 1200 mg/day. The calcium group had a lower incidence of preterm delivery (less than 37 weeks; 7.4% vs 21.1%; p = 0.007); spontaneous labor and preterm delivery (6.4% vs 17.9%; p = 0.01); and low birth weight (9.6% vs 21.1%; p = 0.03). This effect was also present after stratified analysis by level of treatment compliance, urinary tract infection, and chlamydial infection. Life-table analysis demonstrated an overall shift to a higher gestational age in the calcium group compared with the placebo group (log-rank test, p = 0.02). As suggested previously, the observed effect could be mediated by a reduction in uterine smooth muscle contractibility. If confirmed by future research, these results could represent an important preventive intervention for prematurity in high-risk populations.