Newborn's renal function is difficult to assess and its physiology during the first days of life is still incompletely known. Studies suggest that the newborn almost completely reabsorbs creatinine during the first 48 to 72 hours of life, while at the same time it continues to produce its own creatinine. Therefore, the initial stock of creatinine at birth still increases through this production and the non or weak clearance. A better knowledge of renal physiopathology in newborns would allow to improve the therapeutic management of the infants, particularly in case of potential nephrotoxicity. No study has attempted to assess the increase in urinary creatinine excretion in neonates from a given time.

Objectives: To show when urinary creatinine excretion in newborns is efficient. Results: this study mightr show an inflection point in urinary creatinine excretion illustrating the postnatal age when renal function becomes efficient.

Inclusion Criteria:

• All full-term newborns (37WA or more)
• From a physiological and spontaneous pregnancy
• Good neonatal adaptation (Apgar 7 at 5 minutes of life)
• Eutrophic (weight > 10th percentile and < 90th percentile)

Exclusion Criteria:

• Newborns for whom no urine sample is interpretable
• Those with a significant clinical-biological anomaly
• Neonates with hemodynamic disorders such as low blood pressure (median < gestational age) or hypovolemia (shock, signs of dehydration, need for volume expansion) during the first 3 days of life