We studied the sibling similarity in development of covariation among body size characters (body weight (W), body length (S) and head circumference (HC) in Tel Aviv infants from birth up to 2 years of age. We investigated the effects of parental geographic origin, profession, age, current residence, and of family size. Multiple regression analysis with “dummy” procedure established that none of these variables had any significant effect on the aforementioned anthropometric traits. The matrices of phenotypic and genetic correlations, based on sibling similarity data, among age-specific W, S, and HC (measured for 12 different ages) were subjected to principal component analysis in order to elucidate patterns over the age groups. The patterns of both analyses, phenotypic and genetic, were quite similar: one factor in each (first genetic and second phenotypic) had a high positive correlation with early, first 2–3 months of life, W, S, and HC. Three other extracted factors correlated strongly, each with late W, S, and HC, respectively. The results of additional principal component analyses of age-specific W, S, and HC matrices separately, indicate the possibility of existence of two genetic subsystems, the first determining early postnatal stages of human ontogeny, and the second, later phases of child development.