Magnesium is neuroprotective in neonatal animal models of acquired hypoxic-ischemic and/or inflammatory cerebral lesions. It is associated with a significant reduction of perinatal death and cerebral palsy in some observational studies.

The objective of the study is to assess if prenatal magnesium sulfate given to women at risk of preterm birth before 33 week's gestation is neuroprotective.

This is a randomized controlled trial at 18 french tertiary hospitals with stratification by center and multiple births in women at risk of preterm birth before 33 week's gestation and without vascular disease of pregnancy.

Women received 4 g of a 0.1 g/ml magnesium sulfate solution or isotonic serum chloride solution (0.9%).

The main outcome measures are rates of mortality up to discharge, of severe white matter injury (defined by the presence of cavitations and/or intraparenchymal haemorrhages on cranial ultrasonographic studies) and of combined death and severe white matter injury.

The secondary outcome measures are rates of white matter injury (defined by the presence of cavitations and/or intraparenchymal haemorrhages and persisting hypechogenicities at 15 day intervals on cranial ultrasonographic studies), follow-up at two years of age

Inclusion Criteria:

• women pregnant with single, twin or triplet very preterm fetuses younger than 33 week's gestational age if birth was expected or planned within 24 hours

Exclusion Criteria:

• women with vascular disease of pregnancy
• women with severe malformation or chromosomal abnormalities in the fetus
• women with hypotension
• renal insufficiency
• cardiac rhythmic abnormalities
• intake of calcium channel inhibitors
• digitalis or indomethacin less than 24 hours
• persistence of signs of cardiovascular toxicity or tachycardia for more than one hour after cessation of betamimetic intake
• myasthenia
• emergency C section