B, Regression Coefficient; 95% Cis, 95% Confidence Intervals

Appendix Table 1. Regression coefficients for the product term of zygosity and birth weight in the associations between birth weight and BMI, both at individual and within-pair level

Males / Females
Individual level / B / p-value / 95% CIs / B / p-value / 95% CIs
Age 1 / -0.10 / 0.104 / -0.22 / 0.02 / 0.08 / 0.195 / -0.04 / 0.21
Age 2 / -0.06 / 0.331 / -0.19 / 0.06 / 0.07 / 0.343 / -0.07 / 0.21
Age 3 / -0.09 / 0.165 / -0.21 / 0.04 / -0.01 / 0.851 / -0.15 / 0.12
Age 4 / -0.14 / 0.191 / -0.34 / 0.07 / 0.02 / 0.836 / -0.20 / 0.25
Age 5 / -0.16 / 0.126 / -0.37 / 0.05 / 0.00 / 0.975 / -0.21 / 0.21
Age 6 / -0.05 / 0.809 / -0.48 / 0.37 / 0.33 / 0.205 / -0.18 / 0.84
Age 7 / 0.09 / 0.330 / -0.09 / 0.26 / -0.06 / 0.559 / -0.26 / 0.14
Age 8 / -0.28 / 0.029 / -0.52 / -0.03 / 0.28 / 0.087 / -0.04 / 0.59
Age 9 / 0.21 / 0.246 / -0.14 / 0.56 / -0.01 / 0.981 / -0.43 / 0.42
Age 10 / -0.01 / 0.908 / -0.26 / 0.23 / -0.03 / 0.801 / -0.30 / 0.23
Age 11 / -0.12 / 0.446 / -0.44 / 0.19 / 0.14 / 0.489 / -0.25 / 0.53
Age 12 / 0.00 / 0.999 / -0.29 / 0.29 / 0.03 / 0.848 / -0.29 / 0.35
Age 13 / 0.04 / 0.887 / -0.56 / 0.64 / -0.39 / 0.204 / -1.03 / 0.26
Age 14 / -0.08 / 0.728 / -0.50 / 0.35 / -0.06 / 0.765 / -0.48 / 0.35
Age 15 / 0.28 / 0.302 / -0.25 / 0.81 / -0.58 / 0.096 / -1.25 / 0.10
Age 16 / 0.16 / 0.503 / -0.31 / 0.64 / -0.17 / 0.466 / -0.61 / 0.28
Age 17 / 0.42 / 0.038 / 0.02 / 0.82 / 0.03 / 0.880 / -0.36 / 0.42
Age 18 / -0.23 / 0.274 / -0.65 / 0.18 / -0.27 / 0.400 / -0.89 / 0.36
Age 19 / 0.16 / 0.601 / -0.44 / 0.76 / -0.52 / 0.165 / -1.24 / 0.21
Age 20-29 / 0.09 / 0.648 / -0.29 / 0.46 / 0.42 / 0.077 / -0.04 / 0.88
Age 30-39 / 0.36 / 0.223 / -0.22 / 0.93 / -0.38 / 0.151 / -0.90 / 0.14
Age 40-49 / 0.71 / 0.082 / -0.09 / 1.51 / 0.05 / 0.903 / -0.69 / 0.78
Within-pair level
Age 1 / -0.03 / 0.646 / -0.18 / 0.11 / -0.08 / 0.244 / -0.23 / 0.06
Age 2 / 0.12 / 0.125 / -0.03 / 0.28 / -0.15 / 0.058 / -0.30 / 0.01
Age 3 / 0.02 / 0.786 / -0.12 / 0.16 / -0.09 / 0.185 / -0.23 / 0.04
Age 4 / 0.07 / 0.533 / -0.14 / 0.28 / -0.13 / 0.267 / -0.37 / 0.10
Age 5 / 0.10 / 0.376 / -0.12 / 0.32 / 0.10 / 0.428 / -0.14 / 0.34
Age 6 / -0.21 / 0.270 / -0.59 / 0.16 / 0.06 / 0.820 / -0.43 / 0.54
Age 7 / -0.05 / 0.613 / -0.25 / 0.15 / -0.45 / <0.001 / -0.65 / -0.24
Age 8 / 0.08 / 0.591 / -0.22 / 0.38 / 0.12 / 0.473 / -0.21 / 0.46
Age 9 / 0.52 / 0.019 / 0.08 / 0.95 / -0.39 / 0.072 / -0.81 / 0.03
Age 10 / -0.21 / 0.151 / -0.50 / 0.08 / -0.17 / 0.275 / -0.47 / 0.13
Age 11 / -0.18 / 0.336 / -0.55 / 0.19 / -0.13 / 0.519 / -0.54 / 0.27
Age 12 / -0.06 / 0.705 / -0.39 / 0.26 / -0.41 / 0.028 / -0.77 / -0.04
Age 13 / -0.01 / 0.980 / -0.69 / 0.68 / -0.24 / 0.533 / -0.98 / 0.51
Age 14 / 0.01 / 0.961 / -0.50 / 0.53 / 0.07 / 0.789 / -0.44 / 0.58
Age 15 / -0.44 / 0.183 / -1.10 / 0.21 / -0.59 / 0.162 / -1.41 / 0.23
Age 16 / 0.20 / 0.535 / -0.42 / 0.82 / 0.16 / 0.588 / -0.42 / 0.74
Age 17 / 0.08 / 0.783 / -0.50 / 0.66 / 0.13 / 0.631 / -0.40 / 0.65
Age 18 / -0.35 / 0.197 / -0.87 / 0.18 / -0.07 / 0.846 / -0.83 / 0.68
Age 19 / 0.59 / 0.156 / -0.22 / 1.40 / 0.19 / 0.653 / -0.64 / 1.02
Age 20-29 / -0.04 / 0.882 / -0.52 / 0.44 / -0.19 / 0.539 / -0.78 / 0.41
Age 30-39 / 0.53 / 0.209 / -0.30 / 1.35 / 0.02 / 0.960 / -0.76 / 0.80
Age 40-49 / -0.02 / 0.975 / -1.25 / 1.21 / -1.19 / 0.049 / -2.38 / -0.01

In the individual level analyses, BMI was explained by birth weight, zygosity, the product term of zygosity and birth weight, birth year, and twin cohort

In the within-pair level analyses, intra-pair difference in BMI was explained by intra-pair difference in birth weight, zygosity, the product term of zygosity and intra-pair difference in birth weight, birth year, and twin cohort

B, regression coefficient; 95% CIs, 95% confidence intervals

Appendix Table 2- Regression coefficients for the association between birth weight standardized for gestational age and BMI (BMI units per birth weight z-score), with monozygotic (MZ) and dizygotic (DZ) twins treated as individuals

MZ males / DZ males / MZ females / DZ females
Model 1 / Model 2 / Model 1 / Model 2 / Model 1 / Model 2 / Model 1 / Model 2
B / p-value / B / p-value / B / p-value / B / p-value / B / p-value / B / p-value / B / p-value / B / p-value
Age 1 / 0.26 / <0.001 / 0.35 / <0.001 / 0.21 / <0.001 / 0.28 / <0.001 / 0.21 / <0.001 / 0.32 / <0.001 / 0.25 / <0.001 / 0.30 / <0.001
Age 2 / 0.31 / <0.001 / 0.40 / <0.001 / 0.28 / <0.001 / 0.35 / <0.001 / 0.25 / <0.001 / 0.38 / <0.001 / 0.29 / <0.001 / 0.35 / <0.001
Age 3 / 0.30 / <0.001 / 0.38 / <0.001 / 0.25 / <0.001 / 0.30 / <0.001 / 0.24 / <0.001 / 0.32 / <0.001 / 0.24 / <0.001 / 0.26 / <0.001
Age 4 / 0.32 / <0.001 / 0.33 / <0.001 / 0.23 / <0.001 / 0.27 / <0.001 / 0.26 / <0.001 / 0.28 / <0.001 / 0.24 / <0.001 / 0.23 / <0.001
Age 5 / 0.30 / <0.001 / 0.33 / <0.001 / 0.24 / <0.001 / 0.30 / <0.001 / 0.24 / <0.001 / 0.35 / <0.001 / 0.30 / <0.001 / 0.32 / <0.001
Age 6 / 0.45 / <0.001 / 0.32 / <0.001 / 0.08 / 0.526 / -0.02 / 0.900 / 0.36 / 0.010 / 0.36 / 0.021 / 0.44 / 0.014 / 0.57 / 0.001
Age 7 / 0.17 / <0.001 / 0.25 / <0.001 / 0.26 / <0.001 / 0.26 / <0.001 / 0.28 / <0.001 / 0.34 / <0.001 / 0.19 / <0.001 / 0.23 / <0.001
Age 8 / 0.39 / <0.001 / 0.38 / <0.001 / 0.12 / 0.043 / 0.20 / 0.001 / 0.25 / 0.001 / 0.35 / <0.001 / 0.29 / <0.001 / 0.35 / <0.001
Age 9 / 0.19 / 0.030 / 0.31 / <0.001 / 0.28 / 0.002 / 0.33 / <0.001 / 0.42 / <0.001 / 0.56 / <0.001 / 0.32 / 0.005 / 0.42 / <0.001
Age 10 / 0.18 / 0.001 / 0.26 / <0.001 / 0.23 / <0.001 / 0.22 / <0.001 / 0.25 / <0.001 / 0.32 / <0.001 / 0.20 / 0.001 / 0.25 / <0.001
Age 11 / 0.32 / <0.001 / 0.34 / <0.001 / 0.11 / 0.110 / 0.18 / 0.007 / 0.22 / 0.008 / 0.38 / <0.001 / 0.16 / 0.068 / 0.36 / <0.001
Age 12 / 0.21 / <0.001 / 0.26 / <0.001 / 0.27 / <0.001 / 0.26 / <0.001 / 0.18 / 0.007 / 0.32 / <0.001 / 0.20 / 0.003 / 0.20 / 0.003
Age 13 / 0.18 / 0.101 / 0.27 / 0.011 / 0.07 / 0.622 / 0.26 / 0.034 / 0.07 / 0.598 / 0.05 / 0.685 / -0.04 / 0.769 / -0.04 / 0.810
Age 14 / 0.22 / 0.006 / 0.26 / 0.001 / 0.11 / 0.179 / 0.18 / 0.022 / 0.11 / 0.187 / 0.15 / 0.048 / 0.04 / 0.609 / 0.12 / 0.141
Age 15 / 0.11 / 0.317 / 0.15 / 0.199 / 0.20 / 0.069 / 0.12 / 0.253 / 0.35 / 0.008 / 0.45 / <0.001 / 0.22 / 0.155 / 0.25 / 0.070
Age 16 / 0.23 / 0.009 / 0.20 / 0.011 / 0.42 / <0.001 / 0.32 / <0.001 / 0.26 / 0.005 / 0.27 / 0.001 / 0.18 / 0.037 / 0.18 / 0.031
Age 17 / 0.16 / 0.041 / 0.16 / 0.029 / 0.29 / <0.001 / 0.26 / <0.001 / 0.14 / 0.072 / 0.21 / 0.005 / 0.25 / 0.001 / 0.28 / <0.001
Age 18 / 0.13 / 0.082 / 0.12 / 0.066 / 0.03 / 0.730 / 0.11 / 0.171 / 0.29 / 0.010 / 0.45 / <0.001 / 0.14 / 0.285 / 0.21 / 0.048
Age 19 / 0.26 / 0.024 / 0.13 / 0.248 / 0.47 / <0.001 / 0.43 / <0.001 / 0.45 / 0.003 / 0.45 / <0.001 / 0.19 / 0.180 / 0.25 / 0.025
Age 20-29 / 0.21 / 0.008 / 0.16 / 0.026 / 0.20 / 0.016 / 0.16 / 0.032 / 0.01 / 0.941 / 0.12 / 0.222 / 0.18 / 0.116 / 0.25 / 0.017
Age 30-39 / 0.23 / 0.113 / 0.16 / 0.196 / 0.40 / 0.013 / 0.23 / 0.065 / 0.19 / 0.228 / 0.33 / 0.019 / -0.14 / 0.406 / 0.10 / 0.493
Age 40-49 / 0.14 / 0.700 / 0.55 / 0.061 / 0.34 / 0.280 / 0.00 / 0.995 / -0.44 / 0.199 / -0.09 / 0.729 / 0.42 / 0.107 / 0.02 / 0.917

B, regression coefficient

Model 1- Regression model using standardized birth weight as the explanatory variable and BMI as the outcome. Associations were adjusted for birth year and twin cohort.

Model 2- Regression model using gestational age standardized birth weight as the explanatory variable and BMI as the outcome. Associations were adjusted for birth year and twin cohort.

Appendix Table 3- Regression coefficients for the association between PI at birth and later BMI (BMI units per PI unit), both at the individual and within-pair level, in monozygotic (MZ) and dizygotic (DZ) twins

Males / Females
MZ / DZ / MZ / DZ
B / p-value / B / p-value / B / p-value / B / p-value
Individual level
Age 1 / 0.086 / <0.001 / 0.066 / <0.001 / 0.059 / <0.001 / 0.044 / <0.001
Age 2 / 0.065 / <0.001 / 0.076 / <0.001 / 0.047 / <0.001 / 0.047 / <0.001
Age 3 / 0.054 / <0.001 / 0.064 / <0.001 / 0.041 / <0.001 / 0.047 / <0.001
Age 4 / 0.041 / 0.007 / 0.030 / 0.055 / 0.037 / 0.050 / 0.057 / <0.001
Age 5 / 0.051 / <0.001 / 0.057 / <0.001 / 0.049 / <0.001 / 0.067 / <0.001
Age 6 / 0.063 / 0.025 / 0.076 / 0.055 / 0.037 / 0.087 / 0.030 / 0.420
Age 7 / 0.039 / 0.004 / 0.056 / <0.001 / 0.041 / <0.001 / 0.025 / 0.079
Age 8 / 0.053 / 0.001 / 0.020 / 0.345 / 0.027 / 0.128 / -0.015 / 0.592
Age 9 / 0.103 / <0.001 / 0.049 / 0.103 / 0.061 / 0.004 / 0.030 / 0.413
Age 10 / 0.054 / <0.001 / 0.039 / 0.025 / 0.073 / <0.001 / 0.033 / 0.131
Age 11 / 0.099 / <0.001 / 0.054 / 0.032 / 0.069 / 0.001 / 0.019 / 0.541
Age 12 / 0.088 / <0.001 / 0.068 / 0.004 / 0.053 / 0.008 / 0.064 / 0.003
Age 13 / 0.040 / 0.204 / 0.031 / 0.482 / 0.013 / 0.780 / 0.014 / 0.773
Age 14 / 0.103 / <0.001 / 0.046 / 0.175 / 0.053 / 0.035 / 0.059 / 0.055
Age 15 / 0.095 / 0.003 / -0.010 / 0.800 / 0.079 / 0.093 / 0.007 / 0.879
Age 16 / 0.027 / 0.298 / 0.116 / <0.001 / 0.063 / 0.025 / 0.063 / 0.034
Age 17 / 0.119 / <0.001 / 0.088 / 0.001 / 0.074 / 0.004 / 0.052 / 0.045
Age 18 / 0.080 / 0.004 / 0.084 / 0.014 / 0.087 / 0.036 / 0.056 / 0.129
Age 19 / 0.059 / 0.100 / 0.198 / <0.001 / 0.156 / 0.004 / 0.064 / 0.119
Age 20-29 / 0.075 / 0.011 / 0.095 / 0.002 / 0.085 / 0.083 / 0.084 / 0.041
Age 30-39 / 0.056 / 0.496 / 0.159 / 0.049 / 0.001 / 0.985 / -0.065 / 0.432
Within-pair level
Age 1 / 0.076 / <0.001 / 0.106 / <0.001 / 0.076 / <0.001 / 0.093 / <0.001
Age 2 / 0.064 / <0.001 / 0.103 / <0.001 / 0.067 / <0.001 / 0.084 / <0.001
Age 3 / 0.054 / <0.001 / 0.086 / <0.001 / 0.043 / <0.001 / 0.084 / <0.001
Age 4 / 0.021 / 0.022 / 0.051 / 0.001 / 0.052 / <0.001 / 0.073 / <0.001
Age 5 / 0.052 / <0.001 / 0.097 / <0.001 / 0.050 / <0.001 / 0.106 / <0.001
Age 6 / 0.036 / 0.026 / 0.068 / 0.091 / 0.040 / 0.013 / 0.096 / 0.059
Age 7 / 0.031 / <0.001 / 0.051 / 0.001 / 0.050 / <0.001 / 0.031 / 0.055
Age 8 / 0.060 / <0.001 / 0.063 / 0.014 / 0.051 / <0.001 / 0.079 / 0.013
Age 9 / 0.042 / 0.006 / 0.082 / 0.036 / 0.059 / <0.001 / 0.037 / 0.363
Age 10 / 0.050 / <0.001 / 0.042 / 0.085 / 0.053 / <0.001 / 0.104 / <0.001
Age 11 / 0.060 / <0.001 / 0.052 / 0.061 / 0.060 / <0.001 / 0.063 / 0.067
Age 12 / 0.054 / <0.001 / 0.027 / 0.326 / 0.054 / <0.001 / 0.066 / 0.023
Age 13 / 0.084 / 0.005 / 0.050 / 0.323 / 0.034 / 0.187 / 0.037 / 0.575
Age 14 / 0.080 / <0.001 / 0.014 / 0.721 / 0.043 / 0.020 / 0.106 / 0.007
Age 15 / 0.013 / 0.637 / 0.017 / 0.743 / 0.041 / 0.212 / 0.038 / 0.549
Age 16 / 0.096 / <0.001 / 0.142 / 0.001 / 0.043 / 0.024 / 0.085 / 0.055
Age 17 / 0.064 / 0.004 / 0.096 / 0.017 / 0.059 / 0.002 / 0.109 / 0.004
Age 18 / 0.040 / 0.037 / 0.044 / 0.292 / 0.045 / 0.140 / 0.074 / 0.156
Age 19 / -0.008 / 0.801 / 0.137 / 0.016 / 0.095 / 0.002 / 0.162 / 0.009
Age 20-29 / 0.028 / 0.272 / 0.054 / 0.233 / 0.104 / 0.004 / 0.130 / 0.030
Age 30-39 / 0.079 / 0.317 / 0.025 / 0.837 / 0.070 / 0.305 / 0.144 / 0.229

At the individual level, PI at birth was used as the explanatory variable and BMI as the outcome. Associations were adjusted for birth year and twin cohort.

At the within-pair level, intra-pair difference in PI was used as the explanatory variable and intra-pair difference in BMI as the outcome. Associations were adjusted for birth year and twin cohort.

B, regression coefficient