This multisite psychometric measurement study of the BPSN will use repeated measures design. The study will take place in three tertiary care hospitals (Bern, Zurich and Basel) in Switzerland. To examine the impact of GA on pain reaction and its variability, the infants will be stratified into six GA groups ranging from 24 0/7 to 42 0/7. Among preterm infants with GAs up to 36 6/7 weeks, five consecutive heel sticks within the first 14 days of life will be documented. Among healthy-term infants two heel sticks within the first days after birth will be documented. For each heel stick, measurements will be recorded for each of three phases: baseline, heel stick, and recovery. All phases will be videotaped and the child's pain reaction later rated according to the BPSN by five independent raters blinded to the measurement phase. Individual contextual factors of interest for this study are documented in a standardised way in all three participating Neonatal Intensive Care Units (NICUs), and can be extracted from patient charts.