Objective: Normal data for red cell distribution width (RDW) values in newborns are limited and inconsistent; these values have been interpreted according to adult values in most routine blood counts. This study investigated effects of the gestational week on red cell distribution width values in newborns. Materials and methods:The demographic characteristics and complete blood cell count (CBC) values of the babies admitted to our Neonatal Intensive Care Unit between 2017-2019 were evaluated retrospectively. Newborns were categorized as full-term group (gestational weeks ≥37) or preterm group (gestational weeks <37). The mean gestational age, birth weight, sex ratio, hemoglobin (Hb), hematocrit (Hct) and mean corpuscular volume (MCV) values were recorded and RDW values were compared in both groups. Results:The study population included 485 infants, comprising 250 full-term and 235 preterm babies. The mean gestational age of the full-term group was 38.5±1.3 weeks, the mean birth weight was 3198±478g, with 7.2% small for gestational age (SGA) (n=18).The mean gestational age of the preterm group was 32.5±3.4 weeks, the mean birth weight was 2015±815 g, with 5.5% SGA (n=13). The mean RDW values (16.1±1.5) in preterm babies were significantly higher than the RDW values (15.8±1.3) of full-term babies (p=0.011). RDW values of SGA babies were higher in both groups, but there was no significant difference (p>0.05). Conclusion:Red cell distribution width (RDW), an index of the differential diagnosis of neonatal hematologic diseases, has recently been found to be associated with inflammation. In our study, RDW values were statistically higher in preterm babies than in full-term babies. These findings may be useful in the differential diagnosis and prediction of neonatal diseases, together with other blood count parameters.