Objective: The aim of this study was to improve the definition of birth weight discordancy based on its distribution in the population after a correction in the unlike sex pairs.
Study design: The analysis was conducted on 849 Italian twin pairs. Male's birth weight in the unlike sex pairs was corrected by the physiological birth weight difference estimated on 21.595 Italian twins data. Birth weight discordancy was computed as a percentage of the larger twin's birth weight and it was compared by sex pairs and by gestational ages (Kruskal-Wallis test). Kolmogorov-Smirnov test was used to fit a normal distribution to the square-root transformed birth weight discordancy.
Results: The estimated physiological birth weight difference between males and females was 4.39%, without any difference among gestational ages. The 90th centile of birth weight discordancy was 23.9% and the 95th centile was 29.2%.
Conclusions: Ninetieth and 95th centiles of the fitted distribution are proposed as cut-off values for the definition of mild and severe birth weight discordancy.