Infant mortality among singletons and twins in Japan during 1999-2008 on the basis of risk factors.

The infant mortality rate (IMR) among single and twin births from 1999 to 2008 was analyzed using Japanese Vital Statistics. The IMR was 5.3-fold higher in twins than in singletons in 1999 and decreased to 3.9-fold in 2008. The reduced risk of infant mortality in twins relative to singletons may be related, partially, to survival rates, which improved after fetoscopic laser photocoagulation for twin - twin transfusion syndrome. The proportion of neonatal deaths among total infant deaths was 54% for singletons and 74% for twins. Thus, intensive care of single and twin births may be very important during the first month of life to reduce the IMR. The IMR decreased as gestational age (GA) rose in singletons, whereas the IMR in twins decreased as GA rose until 37 weeks and increased thereafter. The IMR was significantly higher in twins than in singletons from the shortest GA (<24 weeks) to 28 weeks as well as ≥38 weeks, whereas the IMR was significantly higher in singletons than in twins from 30 to 36 weeks. As for maternal age, the early neonatal and neonatal mortality rates as well as the IMR in singletons were significantly higher in the youngest maternal age group than in the oldest one, whereas the opposite result was obtained in twins. The lowest IMR in singletons was 1.1 per 1,000 live births for ≥38 weeks of gestation and heaviest birth weight (≥2,000 g), while the lowest IMR in twins was 1.8 at 37 weeks and ≥2,000 g.