A nationwide investigation on gestational age specific birthweight and mortality among Japanese twins.

Gestational age specific birthweight has been used as an important indicator to identify the health status of neonates and to provide appropriate neonatal care services to those at high risk. To identify the gestational age specific birthweight of twins born in secondary and tertiary hospitals in Japan and to explore factors affecting mortality, we used a nationwide obstetric database in Japan. Liveborn twins from 2001 to 2005 in the database, which was established with the national Perinatal Health Care Project, were enrolled (n=22,240). Obstetric data from the medical records were collected and analysed. Approximately 60% of liveborn twins were delivered between weeks 33 and 38, most frequently at 36 and 37 weeks. The mean birthweight of liveborn twins was 2084 g and the mean gestational age was 34.9 weeks. Intrauterine growth differed by parity, mode of delivery, birth order, chorionicity and sex. Using logistic regression, lower gestational age, small for gestational age, monochorionic twins and vaginal delivery tended to have higher risks of mortality.