Objectives: This study measured the impact of shifts in maternal age and parity on the increase in the low birth weight (LBW) rate in Korea. Methods: We obtained raw data for all 6 397 945 live births registered at the Korea National Statistical Office between 1995 and 2005. We calculated the proportion of increment in the LBW rate due to changes in the distribution of maternal age and parity (AP-dis) and the proportion due to changes in the age- and parity-specific LBW rate (AP-spe). Results: The LBW rate increased from 3.02% in 1995 to 4.28% in 2005. The multiple birth rate increased from 1.32% to 2.19% during the same period. Of the 1.26% points increment in the LBW rate, 0.64% points occurred among singleton births and 0.62% points occurred among multiple births. Changes in the AP-dis accounted for 50% of the increase in the LBW rate among singleton births, but did not contribute to the increase in the LBW rate among multiple births. The remainder of the total increment in the LBW rate was explained by the increase in the AP-spe. Conclusions: This study demonstrated that shifts in maternal age and parity among singleton births and increased multiple births were important contributors to the increment in the LBW rate. This study also revealed that the increase in the AP-spe was an equally important contributor as the shifts in maternal age and parity to the increment in the LBW rate among singleton births and was a major contributor among multiple births.