Perinatal & Pediatric
Longitudinal twin growth trajectories, are twins destined to be smaller or do they catch up? Jessica L Gleason* Jessica Gleason Rajeshwari Sundaram Kathryn A. Wagner Katherine L. Grantz
Background: Twins are smaller than singletons at birth. Mounting evidence suggests that intrauterine growth in twins is adaptively different from that of singletons to maximize survival in an intrauterine setting of shared resources. At question is whether twins catch up in growth to that of singletons and at what age. Few studies have evaluated twin growth longitudinally, and none have evaluated twin growth trajectories relative to singletons from birth through adulthood.
Methods: Using data from the National Longitudinal Survey of Youth – 1979 and Child and Young Adult, we compared growth trajectories of twins (n=254) and singletons (n=11,291) from birth through age 20 years. Height and weight were collected every two years from birth through the latest age at interview in 2020. We calculated and compared predicted means for each year of age using linear mixed effects models with quadratic and cubic terms for child age in months, cubic splines for age, and a random intercept and slope, including a nested intercept by family to account for clustering between twins and other siblings. Models were adjusted for child sex and birth year.
Results: Twins were smaller in height, weight, and BMI at birth, appearing to “catch up” at various ages. Differences in height ranged from 3.1 cm (95% CI 2.2, 4.0) at birth to 0.86 cm (0.1, 1.7) at age 8, with the difference becoming 0 by age 12 (β=0.03, -0.98, 1.04). For weight, twins remained from 1.1 kg (0.2, 1.9) smaller at age 5 to 2.0 kg (0.2, 3.9) smaller at age 13, with a 0.6 kg (-2.7, 3.8) difference persisting to age 20. Twin BMI remained smaller through age 16 (β=0.8, 0.1, 1.5), with a small difference remaining at age 20 (β=0.3, -0.8, 1.4).
Conclusions: Although twins caught up to singletons in height by age 12, they remained slightly smaller in weight and subsequent BMI through adolescence. Persistent differences in weight may reflect differential accumulation of fat tissue, which may begin early in utero.