Objective: To evaluate variations in psychological stress, as well as cortisol and prolactin (PRL) levels, in mothers of preterm newborns (NBs) by examining the changes between the initial days of their child’s admission to a neonatal intensive care unit (NICU) and the time prior to hospital discharge. Materials and methods: A quantitative study using a non-experimental, cross-sectional, descriptive survey design with a comparative approach. The sample consisted of 35 mothers of preterm NBs, selected through purposive sampling. The inclusion criteria required that infants be hospitalized in an NICU due to prematurity (< 1500 g birth weight and < 32 weeks of gestation) and that mothers attend the breastfeeding center (BC) to express milk to support breastfeeding. Mothers with a history of psychiatric disorders, as reported during pre-study interviews, were excluded. Results: Marke...