In the neonatal period, the human kidney is characterized by a functional immaturity responsible for an impaired ability to regulate water and sodium homeostasis, which is exacerbated by prematurity. This altered sodium handling could be related to a partial renal aldosterone resistance. Renal sodium reabsorption and potassium excretion are mainly controlled by aldosterone, after binding to the mineralocorticoid receptor (MR). The investigators have analyzed MR expression throughout human and mouse renal development, and the investigators found a weak MR expression at birth. The investigators have conducted a pilot study in full-term newborns, which confirmed a partial neonatal aldosterone resistance. This study also highlighted that urinary aldosterone is the best index to accurately assess aldosterone sensitivity at birth.