 Extreme prematurity affects about 2 of births per year in France and is subject to a significant morbidity and mortality. It is likely that the fluid and electrolyte imbalances associated with mineralocorticoid adrenal insufficiency transient observed in this population of vulnerable newborns contribute to the occurrence of complications that will influence the prognosis medium and long term these children. The expected impact of our pilot study is a direct benefit to the patient, with reduced kidney soda losses from the 3rd day of life and throughout the first week of life assessed by a non-invasive method: urine collection to compress and measurement of urinary Na creatinine. This physiological approach substitution of the deficient hormone allow better control of sodium and water balance. This could limit a number of common complications of extreme prematurity, occurring in the first weeks of life, such as patent ductus arteriosus, intra-ventricular hemorrhage and bronchopulmonary dysplasia. 