Ahlqvist VH, et al. - By performing this prospective cohort analysis, researchers investigated if the birth weight, within the at-term range, is related to later cardiorespiratory fitness (CRF), and if such a link, if exists, is explained by shared and/or nonshared familial factors. Participants were young male adults (n = 286,761) and a subset of siblings (n = 52,544) born at-term. Findings revealed a strong link between birth weight and increasing CRF in young adulthood among men born at-term, across all classes of body mass index. The factors that were not shared between siblings seemed to be the main drivers of this link. Thus, the determination of CRF already in utero may be possible to some extent. A feasible means of raising adult CRF and health can be the prevention of low birth weight, also within the at-term-range.