Perinatal Mortality Rates and Risk Factors for Mortality among Zygotic Twins and Singletons in Japan, 1995-2008

Objective: We aimed to determine the perinatal mortality rates (PMRs) for monozygotic (MZ) twins, dizygotic (DZ) twins, and singletons, together with the associated risk factors for these PMRs. Study design: PMRs of zygotic twins and singletons were estimated using Japanese vital statistics from 1995 to 2008. Results: Declines were seen in fetal death rates [FDRs; defined as deaths after a gestational age of (GA) 22 weeks], early neonatal death rates (ENDRs), and PMRs from 1995 to 2008 to approximately 1/4–1/3 for DZ twins and to 1/2 for both MZ twins and singletons. ENDRs and PMRs were the lowest at maternal ages (MAs) of 30–34 years for MZ and DZ twins and at 25–29 years for singletons. Each mortality rate in singletons was significantly lower at 25–29 years compared with those at other MAs. PMRs were significantly higher for MZ and DZ twins than for singletons in each MA group, except when MA was ≥40 years for DZ twins. PMR was the lowest at GA of 37 weeks for both MZ (6.6) and DZ (3.0) twins but was the lowest at GA of ≥40 weeks in singletons (1.1). PMRs were higher for both MZ and DZ twins than for singletons, except for GA of <36 weeks. PMR was significantly higher for MZ twins than for DZ twins for all GAs, except when GA was ≥39 weeks. The recent increase in preterm birth (i.e., GA of <37 weeks, excluding fetuses delivered at GA of <22 weeks) was associated with a reduction in PMRs for both MZ and DZ twins. Conclusion: In this Japanese population, PMRs decreased for zygotic twins and singletons between 1995 and 2008. The most marked decline was for DZ twins.