Objectives: This study was conducted to investigate possible factors associated with the recent decrease in birth weight in Kumamoto Prefecture between 1979 and Methods: Vital statistics on birth between 1979 and 1997 were obtained from the records of the Ministry of Health and Welfare, Japan, on the basis of which, infants were divided into low-birth weight (LBW:<2500g) group and the normal-birth-weight (≧2500g) group, and the proportions of birth and birth weights by gestational age (from 22

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... o 42 weeks) of both groups between 1979 and 1997 were compared. Results: For the normal-birth-weight group, the peaks of the proportions of birth and birth weights by gestational age in 1997 shifted to early gestation, compared with the peaks in 1979. The birth weight by gestational age in 1997 decreased, compared with that in 1979 among the normal-birth-weight group, in particular, the birth weight from 37 to 40 gestational ages decreased significantly, and the mean birth weight showed a linear relationship with these gestational ages as observed in normal fetal development. However, in the LBW group, the mean birth weight showed a small increase after 1992 and a large increase in birth weight by gestational age in the last month of pregnancy in 1997. Conclusions: The changes in the proportions of birth and birth weights by gestational age among the normal-birth-weight group were found to be the important factors the recent decrease in birth weight. These factors might be associated with advances in medical technology and the fact that young women diet in order to slim.