Catch-Up Growth in Twins: The Influence of Chorionicity and Zygosity from Birth to School Age

This study evaluated the growth trajectories of twins from birth to school age, focusing on weight and height <i>z</i> scores of children born in São Paulo. The longitudinal study comprised 188 healthy twins (<i>M</i> = 9.13 years; <i>SD</i> = 2.16). Weight data were obtained from birth records and reassessed at school age, and height at school age. Singleton birth weights and standard growth curves were used as reference points. Overall, the proportion of children with <i>z</i> scores below the median decreased significantly, from 98.3% at birth to 31.4% at school age. At birth, the dispersion of <i>z</i>-scores below the median was significantly greater among monochorionic compared to dichorionic twins (2.051 vs. 1.701), but this difference was no longer observed at school age (1.861 vs. 0.976). In total, 68.8% of the twins showed improved weight development, with no cases of deterioration. While monozygotic (MZ) and dizygotic (DZ) twin pairs exhibited comparable intertwin birth-weight differences, by school age MZ pairs exhibited more similar weights, whereas DZ pairs showed significantly greater weight discrepancies. For height at school age, mean height <i>z</i> scores were within the expected range for singletons. No significant differences were observed between monochorionic and dichorionic twins, whereas MZ pairs showed smaller within-pair height differences than DZ pairs. These findings suggest that although chorionicity plays an important role during the perinatal period - particularly due to the lower birth weights observed among monochorionic twins - its influence diminishes over time. By school age, weight and height differences are primarily determined by zygosity.