Recent trend of the decrease in mean birthweights of infants in the western provinces of Gifu Prefecture, Japan.

The mean birthweight has decreased recently in Japan. The author assessed the etiology of this trend by comparing the birthweight during the years 1983-1984 and 1993-1994 in Central Japan. All birth certificates of 3767 and 3423 infants were available for 1983-1984 and 1993-1994, respectively. The mean birthweight decreased from 3165 g to 3128 g. The proportion of first infants whose mean birthweight decreased more than that of the other infants increased from 41.0% to 47.1%. Comparison and multivariate analysis revealed the main cause of decreased birthweight as a decrease of the mean birthweight of first infants and changes of the proportion of birth order. Influences of the increases in the proportion of multiple births and that of low birthweight infants to the decrease of mean birthweight were small. The mean birthweight of infants who were born in public hospitals was lower than that of infants born in clinics, private hospitals, and others. The proportion of low birthweight infants was also highest among the infants born in public hospitals. These results indicate that pregnant women with high risk factors are more likely to be referred to the care of public hospitals.