A Cochrane review [Abstract] 1 included 27 studies, all of good quality, with a total of 18 064 women. With calcium supplementation (1g or over/day) compared to placebo, the risk of high blood pressure (RR 0.65, 95% CI 0.53 to 0.81, statistical heterogeneity I2 =74%; 12 trials, n=15 470) and the risk of pre-eclampsia (RR 0.45, 95% CI 0.31 to 0.65, statistical heterogeneity I2 =70%; 13 trials, n=15 730) were reduced. The effect was greatest for high-risk women (RR 0.22, 95% CI 0.12 to 0.42; 5 trials, n=587), and those with low baseline calcium intake (RR 0.36, 95% CI 0.20 to 0.65, statistical heterogeneity I2 =76%; 8 trials, 10 678 women).
The average risk of preterm birth was reduced in the calcium group overall (RR 0.76, 95% CI 0.60 to 0.97; statistical heterogeneity I2 =60%; 11 trials, n=15 275) and amongst women at high risk of developing pre-eclampsia recruited to four small trials (RR 0.45, 95% CI 0.24 to 0.83; n=568). There was no overall effect on the risk of stillbirth or death before discharge from hospital (RR 0.90, 95% CI 0.74 to 1.09; 11 trials, 15 665 babies). The composite outcome maternal death or serious morbidity was reduced (RR 0.80, 0.65 to 0.97; 4 trials, n=9 732). Most of the women in these trials were low risk and had a low calcium diet. Maternal deaths were reported in only one trial. One death occurred in the calcium group and six in the placebo group, a difference which was not statistically significant (RR 0.17, 95% CI 0.02 to 1.39).
Comment: The quality of evidence is downgraded by inconsistency (variability in results across studies).
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