Purpose; To observe the change of mean birth weight over 12 years (2004 – 2015) in all live births, and to evaluate the demographic factors affecting birth weight in a crude population and in singleton term live births delivered at 37 – 41 weeks. Methods; We used data from the Korean Statistical Information Service. After exclusion of unknown data for birth weight, gestational age and maternal age, a total of 5,386,123 births with gestational age from 22 to 43 weeks were analyzed on a yearly basis over 12 years or over the three periods of four years (period 1 to 3). Results; Mean birth weight in a crude population decreased from 2004 to 2015 by 4.79 (standard error, 1.43 ) g per year on average ( P 0.05). Mean gestational age decreased by 0.29 (standard error, 0.05) day per year ( P 0.05), and maternal age increased by 0.20 (standard error, 0.01) year per year ( P 0.05). Proportion of infants who were small for their gestational ages decreased over this time period form 9.24% to 7.14%, and proportion of infants who were large for their gestational age decreased from 11.09% to 6.59%. Rates of twin births, female gender, and unmarried pregnancy increased over time, and the number of births over the second declined. Mother’s education level improved. In multivariate analysis, gestational age was the most significant contributor to birth weight change over time. Adjustment for gestational age attenuated this reduction of birth weight over time, and additional adjustment for other factors minimized this reduction of birth weight over time. Stratification of birth weight by gestational age group showed that reduction of birth weight over time was most significant in term-infants (37 -41 weeks). In singleton term births, birth weight also decreased over three time periods, and proportion of births with early term gestation (37 and 38 weeks) increased from 26.38% to 34.51% over three time periods. Adjustment for gestational age and stratification of mean birth weight by gestational age in singleton term births almost eliminated this decline in birth weight over time. Conclusion; Decline in mean birth weight over time in births may be due to a burden of accumulating risk factors. Gestational age may be the most significant contributor to birth weight reduction, particularly, in singleton term live births.