BACKGROUND

Twin and singleton growth might be adaptively different to maximise survival in a shared intrauterine environment, translating to smaller average twin birthweight. We aimed to compare growth trajectories and risks of overweight and obesity between twins and singletons.

METHODS

We leveraged height, weight, and BMI data from two large and ongoing nationally representative cohorts, the Millennium Cohort Study (MCS) and the National Longitudinal Survey of Youth-Child and Young Adult (NLSY-CYA), that have prospectively collected data from children born in the UK between Sept 1, 2000, and Jan 11, 2002, and participants born in the USA between December, 1970, and January, 2008, respectively, every 2-3 years from birth to adulthood. In each cohort and for a sample combined from both cohorts, we compared growth trajectories of twins and singletons from birth to age 18 years using adjusted predicted means for height, weight, and BMI each year of age derived from mixed models with a cubic function for child age (months), random intercept and slope for age, and a nested intercept for family identity to account for twin clustering. We stratified analyses by sex and by gestational age at delivery and evaluated overweight and obesity risk overall using Poisson regression models with robust error variance. Overweight and obesity were defined using age (months) and sex-specific BMI cut-points from the International Obesity Task Force.

FINDINGS

MCS included 18 967 singletons and 502 twins with at least one height or weight measurement. NLSY-CYA included 11 133 singletons and 246 twins with at least one height or weight measurement. The full sample combined from the two cohorts included 748 twins and 30 106 singletons with measurements up to age 18 years. Twins were shorter than singletons by between 1·22 cm (95% CI 0·69-1·75) at age 3 years and 1·08 cm (0·10-2·05) at 18 years. Twins weighed less than singletons by between 0·82 kg (95% CI 0·49-1·16) at 3 years and 1·93 kg (0·46-3·40) at 18 years, with 0·44-0·76 kg/m lower BMI during this age range. Differences in growth remained when accounting for gestational age at delivery. Analysis of growth trajectories stratified by sex suggest that female twins are catching up with female singletons in late adolescence, with no height difference remaining between female twins and female singletons by age 18 years. At any time between ages 3 and 18 years, twins had lower risk of overweight (adjusted risk ratio 0·78 [95% CI 0·69-0·88]) and obesity (0·77; [0·61-0·96]) than singletons.

INTERPRETATION

Twins from UK and USA cohorts do not catch up with singletons in height or weight but became more similar in BMI by age 18 years. Lower risk of overweight or obesity in twins should be investigated further in the context of cardiometabolic health.

FUNDING

None.