This study aimed to verify the dietary adequacy of full enteral feeding in preterm newborns (PTNB) and its relationship with birth weight (BW) during the period of hospitalization in the neonatal intensive care unit (NICU). This is a prospective cohort study whose population were babies born at less than 37 gestational weeks and weighing less than 2500 g, admitted to a NICU. PTNB were monitored regarding their dietary evolution, considering parenteral and enteral nutrition and adequacy of diet supply in terms of volume, energy, and protein. For statistical analysis, the Statistical Package for the Social Sciences (SPSS) software was used, considering p < 0.05 as significant. A total of 76 PTNB were included. The mean time of using parenteral nutrition was 14 days. The mean time to reach the full enteral feeding for nutrition (FEF-N) was 29 days. However, half of the PTNB reached nutritional recommendations after this means. The time to achieve FEF-N was influenced by BW. Additionally, BW significantly influenced the length of stay in the NICU (p < 0.001). When reaching the recommended full enteral feeding for hydration (FEF-H), 60% of the sample was unable to reach the recommended energy and protein intake. BW influenced the time needed to reach the FEF-H and FEF-N. The lower the BW, the longer it took to achieve dietary adequacy. Despite achieving the FEF-H, most premature babies did not reach the necessary energy and protein intake at the appropriate time.