This study evaluated associations between early neonatal experiences during neonatal intensive care unit (NICU) hospitalization and the development of pain sensitivity, measured by flexion withdrawal reflex (FWR) thresholds, through 18–24 months corrected age in preterm infants. This longitudinal study (2017–2022) in Northeast U.S. specialized level III and IV NICUs monitored preterm infants for approximately 4–6 weeks for NICU pain/stress exposure (using the NICU Infant Stressor Scale - NISS) and analgesic use, assessing FWR thresholds post-discharge at 1, 4, 8–12, and 18–24 months of corrected age. 122 very preterm infants (63.1% male, 82.0% non-Black, 70.5% non-Hispanic) were enrolled, with a mean gestational age of 28.2 ± 2.4 weeks. The mean daily weighted NISS was 90.9 ± 19.9 (acute: 70.7; chronic: 20.2). FWR thresholds declined over time (1.11–0.7 g), suggesting increased population-level sensitivity. However, higher individual cumulative pain/stress exposure at NICU was associated with higher FWR thresholds (lower sensitivity) over time (β = 0.039, p = 0.042). The longitudinal effect of the pain/stress exposures (NISS score) on pain sensitivity (FWR) increased markedly over time in Black females, but was not observed in the Black male, Non-Black female, and Non-Black male subgroups (p < 0.05). Neonatal pain/stress exposure appears to reprogram long-term pain sensitivity development. These findings underscore the critical need for optimized pain management and tailored neuroprotective strategies for all high-risk infants.

The FWR thresholds reflect the maturation of the nociceptive system in preterm infants from birth through 18–24 months of age. Neonatal cumulative pain/stress exposure is associated with altered maturation of FWR thresholds.