Aim: To validate the cutoffs for percentage of weight loss associated with the risk of hypernatremia, taking into account other risk factors. Materials and methods: A total of 50 healthy neonates with a gestational age of ≥35 weeks in the postnatal ward were included. Their weight was checked daily until the 5th day of life using the same scale. Two groups were formed: one with newborns given exclusive direct breastfeeding until the usual cutoff for dehydration (10%), and another with newborns on direct breastfeeding with a cutoff considered at 7%. When the cutoff was exceeded, routine neonatal screening was performed, including monitoring sodium levels. Results: There were 50 cases of hypernatremia. For hypernatremic infants, the mean percentage of weight loss was 6–8%. Associated variables included greater weight loss, male sex, lower education level, primigravida, and cesarean delivery. Conclusion: Screening for early hypernatremia should consider multiple risk factors in addition to the percentage of weight loss.