Magnesium supplementation in pregnancy

Many women, especially those from disadvantaged backgrounds, have intakes of magnesium below recommended levels. Magnesium supplementation during pregnancy may be able to reduce fetal growth retardation and pre-eclampsia, and increase birth weight.

The objective of this review was to assess the effects of magnesium supplementation during pregnancy on maternal, neonatal and paediatric outcomes.

We searched the Cochrane Cochrane Controlled Trials Register. The date of the last search was June 2001.

Randomised and quasi-randomised trials of dietary magnesium supplementation during pregnancy.

Suitability for inclusion and methodological quality were separately assessed by each reviewer. Data were independently extracted by the two reviewers.

Seven trials involving 2689 women were included. Six of these trials randomly allocated women to either an oral magnesium supplement or a control group, whist the largest trial with 985 women had a cluster design where randomisation was according to study centre. The analysis was conducted with and without the cluster trial. In the analysis of all trials, oral magnesium treatment from before the 25th week of gestation was associated with a lower frequency of preterm birth, (relative risk (RR) 0.73, 95% confidence interval (CI) 0.57 to 0.94), a lower frequency of low birth weight (RR 0.67, 95% CI 0.46 to 0.96) and fewer small for gestational age infants (RR 0.70, 95% CI 0.53 to 0.93) compared with placebo. In addition, magnesium treated women had less hospitalisations during pregnancy (RR 0.66, 95% CI 0.49 to 0.89) and fewer cases of antepartum haemorrhage (RR 0.38, 95% CI 0.16 to 0.90) than placebo treated women. In the analysis excluding the cluster randomised trial, the effects of magnesium treatment on the frequencies of preterm birth, low birth weight and small for gestational age were not different from placebo. Of the seven trials included in the review, only one was judged to be of high quality. Poor quality trials are likely to have resulted in a bias favouring magnesium supplementation.

There is not enough high quality evidence to show that dietary magnesium supplementation during pregnancy is beneficial.