OBJECTIVE: Understanding of twin growth in the United States (US) is based on outdated or predominantly non-Hispanic White samples, and the age at which twins catch up to singletons is unclear.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.

We estimated the degree to which language used in the high-profile medical/public health/epidemiology literature implied causality using language linking exposures to outcomes and action recommendations; examined disconnects between language and recommendations; identified the most common linking phrases; and estimated how strongly linking phrases imply causality. We searched for and screened 1,170 articles from 18 high-profile journals (65 per journal) published from 2010-2019. Based on written framing and systematic guidance, 3 reviewers rated the degree of causality implied in abstracts and full text for exposure/outcome linking language and action recommendations. Reviewers rated the causal implication of exposure/outcome linking language as none (no causal implication) in 13.8%, weak in 34.2%, moderate in 33.2%, and strong in 18.7% of abstracts. The implied causality of action recommendations was higher than the implied causality of linking sentences for 44.5% or commensurate for 40.3% of articles. The most common linking word in abstracts was "associate" (45.7%). Reviewers' ratings of linking word roots were highly heterogeneous; over half of reviewers rated "association" as having at least some causal implication. This research undercuts the assumption that avoiding "causal" words leads to clarity of interpretation in medical research.