Abstract
Using vital statistics in Japan (1995-2008), 154,578 live-born twin pairs (128,236 monozygotic [MZ] and 180,920 dizygotic [DZ]) were identified. The proportion of severe discordance among live-born twin births was twice as high in Japanese than Caucasian infants. There were 1858 MZ and 1620 DZ infant deaths. Computation of the relationship between infant mortality rate and birth weight discordance among the twins was performed. Discordance levels were classified into seven groups: <5%, five groups from 5-9% to 25-29%, and ≥30%.The mortality rate was significantly higher in MZ than DZ twins for discordances except at 5-9% and 10-14%. The lowest rate for MZ twins was at 5-9% (7.5 per 1000 live twins) and significantly increased from 10-14% (9.4) to ≥30% (83.4), while the lowest rate for DZ twins was at <5% (6.7), which significantly increased at 10-14% (8.0) and from 25-29% (12.1) to ≥30% (35.5). The relationship was also computed in two gestational age groups (<28 and ≥28 weeks). For births at <28 weeks, three discordances (after 20-24%) in MZ twins were associated with adverse mortality rate. For births at ≥28 weeks, the same relationship was obtained after 10-14% in MZ and after 20-24% in DZ twins. The relationship from 2002 to 2008 showed that the mortality rates significantly increased after 10-14% for both types of twins. In conclusion, five discordance levels in MZ and three levels in DZ twins were associated with adverse mortality rates.
Highlights
According to Imaizumi (2015), the infant mortality rates (IMRs) for monozygotic (MZ) and dizygotic (DZ) twins decreased by one-half from 1995 to 2007 in Japan, where the rate was two times higher in MZ than in DZ twins
The present study examines the relationship between IMRs and birth weight discordance (BWD) levels among MZ and DZ twins
The corresponding proportions were at 5–9% for 25% and 24%, at 10–14% for 17% and 19%, at 15–19% for 10% and 13%, at 20–24% for 6% and 8%, at 25–29% for both 4%, and at ≥30% for both 5%, respectively. These proportions decreased with BWD levels, except at ≥30% for both MZ and DZ twins
Summary
According to Imaizumi (2015), the infant mortality rates (IMRs) for monozygotic (MZ) and dizygotic (DZ) twins decreased by one-half from 1995 to 2007 in Japan, where the rate was two times higher in MZ than in DZ twins. As for maternal ages, IMRs for both MZ and DZ twins were significantly higher at maternal ages of
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