GSTM1 and nutritional disorder: This study aims to (i) evaluate the prevalence of iron deficiency among preterm infants ≤ 29 weeks GA, as defined by Ret-Hb levels ≤ 29 pg, at 36 weeks postmenstrual age (PMA), and compare the perinatal risk factors between preterm infants with and without iron-deficiency; and (ii) examine the association between Ret-Hb levels at 36 weeks PMA and significant neurodevelopmental impairment at 18–24 months corrected age.