Objective: To evaluate the Infant Mortality Rate (IMR) associated with birth defects in singletons and twins and to identify risk factors for IMR.
Study design: IMR was estimated using Japanese vital statistics from 1995 to 2008.
Results: All IMRs associated with birth defects significantly decreased from 1995 to 2008 for both singletons and twins. IMR was approximately 3-fold higher in the latter than in the former during the period 1995–2008. In contrast, IMR with chromosomal abnormalities was the same in singletons and twins. The proportion of infant deaths associated with all birth defects among the total number of infant deaths was nearly constant (40%) during the examination period in singletons. In contrast, the value in twins was 20% in 1995 and increased to 25% in 2008. During the period, IMR significantly decreased in six categories of birth defects in singletons and two categories in twins. The Relative Risk (RR) in twins vs. singletons was 52-fold for anencephaly and 14-fold for patent ductus arteriosus. RR in twins vs. singletons was 2.9-fold for IMR and 3.8-fold for the neonatal mortality rate. With regard to the circulatory system (Q20-Q28), RR of IMR in twins vs. singletons decreased with maternal age (MA) (from 7.6-fold to 2-fold). For chromosomal abnormalities (Q90-Q99), IMR in singletons was 15-fold higher for the oldest MA group than in the youngest group. With regard to the Gestational Age (GA) and birth weight, IMR associated with all birth defects decreased with an increase in GA from the shortest week of GA to the longest and from the lowest birth weight to the highest.
Conclusion: IMR associated with all birth defects decreased significantly from 1995 to 2008 for both singletons and twins. RR in twins vs. singletons was approximately 3-fold during the period 1995-2008. IMR for Q90-Q99 was similar in singletons and twins. IMR of anencephaly in twins increased following in vitro fertilization and stimulation of ovulation. To reduce different IMRs between twins and singletons, it is important to provide intensive care for twin babies during the neonatal period.