As health professionals, we have memorized that "newborns can lose up to 10% of their birth weight during the first week of life and should regain it by two weeks of age". However, this statement, which appears so accurate, comes from studies conducted in the 1960s, when medical knowledge and how newborns were fed were utterly different from what it is today. Currently, multiple factors contribute to the percentage of weight loss at birth and the rate at which this weight is regained. There are nomograms for exclusively breastfed and formula-fed newborns and those by vaginal or cesarean delivery. To meet the World Health Organization's goal of exclusively breastfeeding newborns, it is essential to recognize that "loss of more than 10% of birth weight" does not represent the need for formula supplementation. When assessing these cases, we must consider several factors that influence the percentage of weight loss in newborns. Therefore, diagnostic decisions should always be individualized in favor of breastfeeding.