Population based reference birth weights and risk factors for neonatal death in Japanese singletons and twins

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This study aimed to construct population-based singleton and twin birthweight reference curves, and to identify the risk factors relating to early- and neonatal death. For the birthweight reference, birthweight from birth registry data for 2008-2010 was tabulated by gestational week, and a reference curve calculated using smoothed b-splines. To assess risk factors for early- and neonatal death, multilevel Poisson regression was performed, adjusting for gestational age, maternal age, parity, sex, and birthweight discordance for twins.The birthweight reference curve developed in this study should be used for twins in clinical settings. Young mothers (under 20 years old for singletons and under 25 years old for twins ), babies delivered before the 25th week of gestational age, and as a twin-specific factor pairs with large birthweight difference within the pairs or SGA-concordant pairs for are at high risk for early neonatal mortality.