Early childhood weight trajectory differences in twins, singletons, and gestational-age matched singletons

Objective: Understanding of twin growth in the United States (US) is based on outdated or predominantly nonHispanic White samples, and the age at which twins catch up to singletons is unclear. In this study, we characterized normative weight trajectories of twins and singletons in a contemporary, diverse cohort. Methods: Data were from the PROMISE study, an electronic health record-based cohort of pregnant people and their children in the US (2005-2021). The Jenss model was used to characterize weight trajectories from 0 to 24 months of age. Twins (n = 716) were compared to the full cohort of singletons (n = 40,075) and a matched sample with similar gestational age at birth (GA) (n = 7160). Results: Male and female twins had lower birth weight compared to singletons and experienced a high rate of weight gain throughout infancy. Among males, twins caught up in weight to the full singleton cohort and to GA- matched singletons at approximately 12 and 6 months, respectively. Among females, twins caught up to GA- matched singletons at approximately 15 months but did not fully overcome their birth weight disadvantage to the full singleton sample by 24 months. Conclusions: These findings highlight that the use of singleton growth charts or preterm singleton growth charts among twins may be inappropriate and suggest the need for a twin-specific growth chart. Future research is needed to understand factors that drive differences in weight trajectories between twins and singletons and to guide twin-specific guidelines.