In a randomised double-blind study 568 women were given orally either 15 mmol magnesium-aspartate hydrochloride per day or aspartic acid as placebo. The outcome of pregnancy was significantly improved in the magnesium group (fewer maternal hospitalisations, reduction in preterm delivery and less frequent referral of newborn to the neonatal intensive care unit). In the 437 patients with regular intake of tablets, haematocrit, haemoglobin and calcium show a decrease up to 20 weeks of gestation (wks) and an increase during the last 8 wks, whereas magnesium stays at a low level. Oestriol and HPL are rising steadily during gestation. In all parameters, no differences were found between the magnesium and placebo group, except for a higher level of oestriol at 33 to 36 wks and a lower level above 36 wks in the magnesium group. Because of the numerous benefits throughout the pregnancy magnesium supplementation is recommended for all pregnancies.