[Risk factors for infant mortality from diseases in Japan 1995-98]

Since 1995, additional information (i.e. birth weight, singleton/multiple births, gestational weeks, maternal age, maternal parity and stillbirth experience) has been required for certificates of infant (less than 1 year of age) death from diseases in Japan. The present study examined the effects of biological, demographic and social variables, as reported on birth and death certificates, on infant, neonatal and postneonatal mortality in Japan.

Using data from vital statistics between 1995 and 1998, more than 4,787,000 livebirths and 16,000 infant deaths from diseases were analyzed. Univariate and multivariate analyses with the Poisson regression model were employed to assess the effects of variables on infant, neonatal and postneonatal mortality by singleton and multiple livebirths separately.

The infant mortality rates from diseases were 3.2/1000 for singleton livebirths and 17.7/1000 for multiple livebirths. In singleton livebirths, low birth weight, infant born in earlier years, being a male infant, employment status as "unemployed or unknown", short gestational weeks, late birth in multiparity and maternal stillbirth experience were all significantly related to increased risk of neonatal and postneonatal deaths. Teenage mother were also at high risk of postneonatal deaths. Regional differences were observed. Compared with singleton livebirths, birthweight-specific mortality rates in multiple livebirths were relatively low among infants weighing under 2500 g. In multiple livebirths, elevated risk of death was associated with low birth weight, infant born in earlier years, employment status as "unemployed or unknown" and short gestational weeks. However, late birth in multiparity was related to a reduced risk of death, and maternal stillbirth experience was not a significant variable.

This study provided the first quantitative estimate of risk of infant mortality from diseases in Japan. Since a more detailed elucidation of actual conditions and risk factors of infant deaths by vital statistics has become possible, efficient measures for improvement of infant mortality are to be expected.