[Magnesium and the threat of premature labor]

The magnesium ion, which is a bivalent cation and is necessary for life, is a co-factor in more than 300 enzyme reactions. It has an action on uterine myometrium causing it to relax by stimulating B2 adrenergic receptors and cyclical AMP. It competes with the calcium ion which brings about inhibition of myosine kinase, and therefore a drop in phosphorylated myosine. Deficiency of magnesium is one explanation for uterine overactivity; because it is difficult to estimate how important it is in certain premature labours because this is caused often by many factors, and the levels of plasma magnesium and of blood cell magnesium give poor information about the magnesium pool. The tocolytic properties of magnesium are interesting. One has to distinguish between oral magnesium-therapy, given as a preventive from the first trimester of pregnancy, and parenteral magnesium therapy which can be curative and which, like beta-mimetics, is indicated in threatened premature delivery.