Effect of disparities in birth weight on differences in postnatal growth of monozygotic and dizygotic twins.

Influence of intrapair differences in birth weight (IDBW) on the patterns of postnatal growth of MZ and DZ twins from middle/late childhood to adolescence were studied in 49 MZ, 40 DZ male and 40 MZ, 35 DZ female pairs coming from the Wrocław Longitudinal Twins Study. Intrapair differences in the patterns in postnatal growth were expressed in several indices: average differences in standardized values (ASD Height, ASD Weight); absolute average differences in standardized values (ABSD Height, ABSD Weight); average Euclidean distances coefficient (EUCD Height, EUCD Weight); coefficient of shape differences (SHC Height, SHC Weight); measure of deviation (MD Height, MD Weight) and correlation coefficient for the standardized attained values (CORC Height, CORC Weight). Relationships between IDBW and indices were based on the means of Spearman Rank Order Correlation. Additionally for 40 MZ, 35 DZ male and 25 MZ, 18 DZ female pairs, the relationships between IDBW and intrapair differences in biological parameters derived from Preece Baines model 1, describing differences in time (DT1), height (DH1), velocity (DV1) at the beginning of the adolescent growth spurt, differences in time (DT2), height (DH2), velocity (DV2) at peak height velocity and differences in adult height (DAH) were examined. The results showed that only in MZ girls did dissimilarity in birth weights significantly impair the subsequent growth in stature. Furthermore, the results revealed that birth weight influences the parameter describing adult stature in MZ girls. It is concluded that the lack of relationship in DZ twins is due to their unique genotype, which strongly determines the postnatal growth. Three possible interpretations are given as explanations of the results obtained in this study: male excess neonates mortality, 'third factor' and programmed of growth in utero.