Several studies have demonstrated that vitamin D deficiency at birth is a risk factor of bronchopulmonary dysplasia. However, in an animal model of bronchopulmonary dysplasia vitamin D overdose has also been associated with an increased mortality and an increased lung injury. Such vitamin D overdose has been frequently reported in hospitalized neonates receiving the current supplementation.

The hypothesis is that vitamin D overdose is an independent risk factor of bronchopulmonary dysplasia or death among infants born below 31 weeks gestational age excluding infants with vitamin D deficiency.

This retrospective cohort study will include all infants born before 31 weeks of gestation (WG), who were hospitalized in a tertiary neonatal intensive care unit (NICU) during at least 10 days, for who at least one 25OH vitamin D determination was performed before 36 WG corrected age and whose parents are not opposed to the study. A descriptive analysis of the cohort depending on the occurrence of vitamin D overdose will be performed. A multivariate analysis will determine if vitamin D overdose is an independent risk factor of bronchopulmonary dysplasia or death among preterm infants, adjusting on the covariates known to be associated with bronchopulmonary dysplasia.

Inclusion Criteria:

• Inclusion Criteria * :
• Infant born before 31 weeks of gestation.
• hospitalized in the NICU during at least 10 days
• with at least a 25OH-D determination available before 36 weeks corrected age

Exclusion Criteria:

-Infant with at least a 25OH-D below 50 nmol/L

Infants hospitalized in the NICU at Hôpital Femme Mère Enfant, Bron, France. With a measure of 25OHD each month from the first month of life, as routinely recommended in this NICU.