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OBJECTIVES: Birth weight is a widely used indicator of prenatal experiences in models of the developmental origins of cognitive ability across the lifespan. This study aimed to examine the association between birth weight and cognitive ability using a community sample of twins followed prospectively from infancy to midlife. We leveraged the twin study design to identify phenotypic and biometric associations between the two constructs.METHODS: The sample consisted of 1,501 participants (387 dizygotic pairs, 360 monozygotic pairs, and 7 singletons; 53.1% female; 91.1% White) from the Louisville Twin Study. We modeled the change in the strength of the association between birth weight and cognitive ability using exponential decay functions.RESULTS: The magnitude of the association between birth weight and cognitive ability declined exponentially from infancy (β = .59, p < .05) to midlife (β = .27, p < .05). The lower asymptote of the exponential decay function was reached at about age 2.5 years of age, after which the association between birth weight and cognitive ability stabilized and remained constant up to midlife. A 1-kilogram increase in birth weight was associated with an 8.85-point increase in cognitive scores at 3 months and a 4.05-point advantage after about 2.5 years. Biometric regression models revealed that shared environmental factors accounted for the decline in the association between birth weight and cognitive ability. A small, positive within-pair association persisted into midlife.DISCUSSION: These findings suggest that prenatal experiences may have lasting effects on cognitive development across the lifespan, supporting developmental origin models of cognitive ability.