The influence of intrauterine conditions on later weight gain remains unclear. We examined the associations of birth size characteristics (weight, length, and ponderal index (PI)) with body mass index (BMI) trajectories and fluctuation in adolescence and adulthood using a twin design, which provides insights into the role of genetic and environmental factors.Data from two Finnish twin cohorts including 9850 twin individuals (48 % males) were used. Weight and length or height were measured at birth and at different ages during adolescence and adulthood (11.5-37 years in FinnTwin12; 16-34 years in FinnTwin16). BMI trajectories across different stages of adolescence and adulthood were calculated as the difference in BMI divided by the time elapsed between measurements. BMI fluctuation was assessed as the variance of BMI trajectories at each stage. Linear regression models were used to examine the associations of birth size characteristics with BMI trajectories and fluctuation in adolescence and adulthood. Interactions between baseline BMI and birth characteristics were assessed. Within-pair analysis was performed to assess whether the identified associations persist while controlling for genetic effects.BMI trajectories during early adolescence were positively associated with birth PI and negatively with birth weight and length. BMI trajectories during middle adolescence were positively associated with birth length and negatively with birth weight. PI showed a negative association with BMI trajectories in late adolescence. Moreover, BMI fluctuation in adulthood was negatively associated with birth weight and length. No significant interactions were found between birth size characteristics and baseline BMI in explaining BMI trajectories and fluctuation at different stages of adolescence and adulthood. Among the identified associations, none remained significant in within-pair analysis.Our findings suggest that birth size has a long-term influence on BMI development. However, these associations may not be due to the intrauterine environment but may rather indicate the role of shared genetic factors.