A sample of 681 Israeli boys and girls, including 355 regular siblings (SB), 112 pairs of dizygotic (DZ) and 51 pairs of monozygotic (MZ) twins, was measured for body weight (WT), length (HT) and head circumference (HC) at birth and during the first year of life. The Count model with three parameters was chosen as the best fitting and most parsimonious function to approximate growth of the studied traits. The curves' fitting parameters were estimated for WT, HT and HC for each individual. To test the assumption that there is a genetic source influencing the pattern of growth for each trait, familial correlations between parameter estimates were computed for MZ, DZ twins and SB. In all instances MZ twins showed the highest within-pair correlation in parameters of growth (from 0.58 to 0.86), while SB showed the lowest ones (from 0.10 to 0.70). Variance decomposition analysis was used to simultaneously assess the contribution of gender, gestational age, additive genetic factor, common sibs and common intrauterine environmental effects on total variance of each studied trait separately. All these sources of variation were statistically significant, though the effect of intrauterine environment played a substantial role in early stages of child physical development, explaining from 18.1% to 70.6% of the total variance of the growth curve parameters. Further analyses are needed to clarify how this environment affects child growth and for how long.