 APPROACH: Similar to Aim 1, premature infants with GA 23 to 31 weeks will be prospectively recruited from Kentucky Childrens Hospital Neonatal Intensive Care Unit NICU. High resolution oxygen saturation monitoring will be performed for the first 2 months of life. Urine will be collected daily in the first week of life and then weekly until 2 months of age and hospital discharge. Collecting urine is noninvasive and minimal risk. Samples will be de-identified and stored in the investigators research laboratory. Standardized kits to measure biomarkers associated with kidney injury such as albumin, β2-microglobulin β2MG, cystatin C Cys C, epidermal growth factor EGF, neutrophil gelatinase associated lipocalin NGAL, osteopontin OPN, and uromodulin UMOD will be used. Since IH increases with age and persists during NICU stay, the cumulative effect of intermittent hypoxemia will be correlated with biomarker concentrations weekly throughout the study period and at discharge. 