Associations (β –coefficients; 95% CI) between within-twin pair difference in birth weight (kg) and the within-twin pair difference in adult hand grip strength or fat free mass stratified by sex and zygosity.

Model 1 – unadjusted.Model 2 – adjusted for within twin pair difference in adult height.Model 3 – adjusted for within twin pair difference in adult fat-free mass.Linear regression β coefficients represents the difference in adult grip strength (kg) or fat free mass (kg) per 1 kg difference in birth weight between twin 1 and twin 2 with 95% Confidence Intervals (MZ Monozygotic twins; DZ: dizygotic twins).