Abstract
OBJECTIVE: To investigate the behavior of weight-for-age z-score (WAZ) and associated factors in preterm newborns (PTNB) with very low birth weight (VLBW) and extremely low birth weight (ELBW), comparing them to low birth weight (LBW) during four weeks (wk.) of hospitalization in a neonatal intensive care unit (NICU). METHODS: Longitudinal study conducted using data from PTNB records in the southern Brazil NICU, between January 2017 and December 2020. Non-twin PTNB with gestational age of ≥ 24 and < 37 wk. and a birth weight (BW) ≥ 500 g were included. The outcome was WAZ, and the exposure was the PTNB's BW, categorized as VLBW/ELBW (< 1500 g) and LBW (≥ 1500 g up to 2500 g). Energy (kcal/kg/day) and protein (g/kg/day) intakes were also evaluated. Two-way ANOVA and multiple linear regression were used to assess the association between demographic, clinical, and nutritional factors and the WAZ. RESULTS: The majority of PTNB were male (60 %) and had a birth weight ≥1500 g (65 %). A significant interaction was between the BW category and the length of hospitalization on WAZ (F = 4.0; p = 0.003). In the VLBW and ELBW, the WAZ was significantly lower in the first wk. compared to the LBW [-1.05 (-1.34;-0.75) vs -0.34 (-0.49;-0.18)]. Factors such as male sex, sepsis, initiation of enteral nutrition (EN), and protein intake were associated with WAZ behavior. CONCLUSION: The downward trend of the WAZ curve was associated with the interaction between birth weight and length of hospitalization, especially in PTNB with VLBW and ELBW. The study concluded that male sex and sepsis contributed to the observed decline.