Preterm Birth and Birth Weight and Cognitive Performance Among Males Eligible for Military Service

Importance: Preterm birth and low birth weight are associated with adverse neurodevelopmental outcomes in childhood, but their association with cognitive performance in young adulthood remains incompletely characterized at the population level. Objective: To determine whether gestational age and birth weight are associated with cognitive performance during compulsory military service in young adult males. Design, Setting, and Participants: This nationwide retrospective register-based cohort study included males born in Finland between January 1, 1987, and December 31, 1998 (N = 350 617), identified using linked national health and military registers. The dataset was based on cognitive testing conducted during military service at a mean age of 19 to 20 years. Preconscription deaths and individuals without military cognitive testing data were excluded. Data analysis was performed from June 1 to December 31, 2025. Exposures: Gestational age at birth and birth weight. Main Outcomes and Measures: Cognitive performance was measured using the Finnish Defense Forces' cognitive basic test 1, consisting of verbal, numerical, and visuospatial subtests. Scores were standardized on a 9-point scale (mean [SD], 5.00 [1.96]), with the overall score calculated as the mean of the 3 subtests. Results: A total of 192 767 participants were included, of whom 10 016 (5.2%) were born preterm. Individuals born extremely preterm scored lower on the overall cognitive test compared with full-term individuals (mean difference, -0.78 [95% CI, -1.07 to -0.49] points). The groups born very preterm and moderately preterm showed progressively smaller deficits, while those born late preterm showed minimal differences compared with their peers born full term. Associations reached a plateau at approximately 36 to 38 gestational weeks. Similar dose-response associations were observed across birth weight categories, with individuals with extremely low birth weight scoring 0.89 (95% CI, -1.23 to -0.55) points lower than those with normal birth weight. Differences were most pronounced in visuospatial and numerical subtests, whereas verbal performance showed little or no association with gestational age or birth weight. Conclusions and Relevance: Lower gestational age and birth weight were associated with poorer cognitive performance in young adulthood in a dose-response pattern, with the largest differences observed among individuals born extremely preterm and with very low birth weight. Late preterm birth was associated with minimal cognitive differences. These findings underscore the long-term cognitive impact of preterm birth.