Calcium supplementation commencing before or early in pregnancy, for preventing hypertensive disorders of pregnancy.

This systematic review/meta-analysis included one randomized placebo-controlled trial of parous nonpregnant women with a history of preeclampsia who intended to become pregnant. Participants were enrolled in hospitals in Argentina, South Africa, and Zimbabwe and followed from preconception through pregnancy.

Calcium supplementation consisted of 500 mg elemental calcium as a calcium carbonate tablet taken orally once daily from pre-pregnancy randomization until 20 weeks' gestation, followed by 1.5 mg calcium daily for the remainder of pregnancy. The comparator was a matching placebo sucrose tablet given on the same schedule.

Calcium supplementation before conception and in early pregnancy may reduce the composite outcome of preeclampsia or pregnancy loss, but evidence for most other maternal and neonatal outcomes was inconclusive. For preeclampsia, 69/296 women in the calcium group versus 82/283 in the placebo group had the outcome (RR 0.80, 0.61 to 1.06), and for preeclampsia or pregnancy loss or stillbirth at any gestational age the result was 107/323 versus 126/310 (RR 0.82, 0.66 to 1.00). Other key outcomes were not clearly improved, including severe maternal morbidity and mortality index (63/296 vs 65/283; RR 0.93, 0.68 to 1.26).

Evidence came from a single trial, so certainty is limited and the findings are not robustly generalizable. Several outcomes were underpowered or imprecise, with many confidence intervals crossing no effect. The regimen and population were specific to women with prior preeclampsia who were planning pregnancy, which narrows applicability.