[Decline in birthweight-specific infant mortality rates of singleton livebirths in the 1990s in Japan].
OBJECTIVE
This study investigated the decline in birthweight-specific infant mortality rates in Tohoku, Tokai and Kyushu regions between 1989 to 1990 and 1995 to 1998.


METHODS
Information for both births and infant deaths from any diseases were collected for two periods in the Tohoku, Tokai and Kyushu regions. The first involved a cohort of singleton livebirths (n = 404, 158) in the three regions in 1989, whose birth certificates were linked with infant death certificates. The second was for singleton livebirths (n = 1,503,230) in the same regions between 1995 and 1998, and infant deaths from any diseases were identified based on vital statistics.


RESULTS
Overall infant mortality rate from any diseases declined 16%, from the 3.86/1000 of the first group to the 3.24/1000 of the second group. A shift to a lower birthweight distribution weakened the improvement in the overall mortality rate, so that birthweight standardized mortality rates were reduced more than crude ones. Based on birthweight adjusted relative risk, neonatal, postneonatal and infant mortality rates for each birthweight group were improved by almost uniform ratio, that is 31%, 12% and 23% respectively. Improvement of survival of infants with birthweight of a 2500 or more accounted for most of the decline in the infant mortality rates in the 1990s. Mortality rates from "certain conditions originating in the perinatal period" and "congenital anomalies" decreased remarkably, whereas the sudden infant death syndrome increased.


CONCLUSIONS
We conclude that reduction of mortality rates in the infant period in Japan in the 1990s was attributable to decline in birthweight-specific mortality rates and weakened by a shift to a lower birthweight distribution over time. Normal birthweight infants showed the greatest improvement.
Fujita
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D001724:Birth Weight; D005260:Female; D006801:Humans; D007223:Infant; D007226:Infant Mortality; D007231:Infant, Newborn; D007564:Japan / Q000453:epidemiology; D011247:Pregnancy; D013398:Sudden Infant Death / Q000453:epidemiology