[Secular changes of the mean birth weight in Japan during the past 20 years (single births, 1969-88)].

Using vital statistics (1981-1988) of the Japanese government, the mean birth weight (MBW) classified by sex was calculated from a birth weight distribution of the single births with 500g intervals for all Japan, prefectures and regions. These MBWs were linked with the MBW of 1969-1980 in our previous reports (Ref. 1) 2)). The MBWs of single births for twenty years (1969-1988) were investigated and it was pointed out that the MBWs classified by area (all Japan, prefectures and regions) were on the decrease recently and that the areal differences of the MBW were becoming large. For all Japan a secular change in the MBW was discussed for the term of a standard deviation, a skewness, a kurtosis, and relative frequencies of less than 2500g (weight class 1), those from 2500g to 3999g (weight class 2) and those of 4000g or more (weight class 3). These investigations revealed that birth weights of Japan have had a tendency to have a small standard deviation, a negative skewness, and a large kurtosis. Weight class 1 decreased until 1976 and increased after this year. Weight class 2 was on the increase up to 1988, and weight class 3 increased until 1973 and decreased afterward, as if it were accompanied by the MBW. For the purpose of confirming the contribution of each weight class to the annual change of the MBW, an analytical method for the weight frequency distribution was proposed as follows. Let M, M', pi and pi' represent the MBW of a given year (year A), the MBW of an another year (year B), the relative frequency of the i-th weight class in year A and that in year B, respectively. Suppose that M(i)'' represents the MBW when only the i-th weight class has pi as a relative frequency even in year B. Then, as the result of a fairly simple calculation, the difference between the MBW in year A and that in year B, M'-M, is obtained as follows; M'-M = sigma (1-pi') (M'-M(i)''). Therefore, the amount (1-pi') (M'-M(i)'') is able to be considered as a share of the i-th weight class in relation to the difference of M'-M. Accordingly, the share proportioned to the summation of the absolute amount [formula: see text] may represent the degree of the contribution of the i-th weight class to the difference, M'-M.(ABSTRACT TRUNCATED AT 400 WORDS)