Annual trends of mean birth weight (MBW) at an obstetric facility in the city of Osaka were examined, based on the data in the delivery records for 30 years from January 1962 through December 1991. MBW at this facility had been decreasing during the years of 1975-1985, after being adjusted for parity (primipara/multipara) and the infants'sex. In addition, annual trends of the rate of intrauterine growth and mean gestational period, which are considered to be the two major processes to governing birth weight, were retarded or shortened during almost the same time. Among some possible factors with causal impacts on birth weight, the advance in maternal age, the lowered parity, and the increase in premature delivery may explain these findings. Additionally, it was suggested that the change of socio-economical environment, including the lifestyle of women, may have did not agree with the decreasing trends of MBW.
