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The Effect of Birth Weight Discordance on Infant Mortality Rates Among Zygotic Twins in Japan, 1995-2008

Y. Imaizumi

Infant mortality rates (IMRs) of monozygotic (MZ) and dizygotic (DZ) twins were estimated using vital statistics from Japan between 1995 to 2008. Using the same data, Imaizumi [1] reported that mortality risk factors were maternal ages of <20 years and gestational ages of up to 35 weeks. In the present study, 128,236 MZ and 180,920 DZ twins were used as denominators to compute IMRs in zygotic twins. Numbers of infant deaths were 1,858 MZ and 1,620 DZ twins. Birth weight discordance (BWD) levels were classified into seven groups from <5% to 30%≥. Fig. 1 shows the relationship between IMRs and BWD levels. The lowest IMR was 7.5 per 1000 live births at 5−9% in MZ and 6.7 at <5% in DZ twins. IMRs were significantly higher in MZ than DZ twins except two BWD levels from 5%−9% to 10−14%. The lowest IMR in MZ twins was significantly increased after 10−14%. The lowest IMR in DZ twins was 6.7 at <5% and significantly increased at 10−14% and after 25−29%. As for gestational age (GA) <28 weeks, the ratios of the highest vs. the lowest IMRs were 2.2(376.2/173.6) for MZ and 1.3 (275.2/207.2) for DZ twins. As for 28 weeks GA, the corresponding ratios were 13.8 (53.7/3.9) vs. 9.1 (29.1/3.2), respectively. Namely, under GA 28 weeks, a risk factor of BWD was not a main factor.