Percentile charts of twin birthweight.

BACKGROUND: The birthweight of multiples is naturally lower than that of singletons. Given that the incidence of twin pregnancy has risen in recent years, it seems reasonable to create standards of birthweight separately for twins. This could help in the objective assessment of small and large for gestational age twin newborns. The main goal of this study was therefore to construct and present up-to-date birthweight references.

METHODS: The present percentile charts for twins are based on a cohort retrospective study of 757 pairs of twins (767 boys and 709 girls) born between weeks 25 and 39 of gestation. Mean and standard deviation were calculated for the subsequent weeks of gestation. Percentiles were read for the subsequent gestational age. The obtained curves were smoothed with a fifth-degree polynomial function. The significance of differences between the 50th percentile values for twins and singletons was estimated using median test.

RESULTS: In both sexes, a continuous observable trend occurs of a significantly lower average birthweight for twins. Differences increase with increasing gestational age and are greater in girls. The estimated 50th percentile for twins was greater than the estimated 10th percentile for singletons. This supports the notion of discordant growth as a physiological adaptation that promotes maturity.

CONCLUSIONS: Percentile charts for singletons are not applicable for twins. This indicates the importance of applying separate percentile charts for twins, enabling objective evaluation of their health status and identifying deviations from normality.