Objectives We estimated seasonality in birthweight over time and assessed how seasonality changed. Methods We analyzed all full‐term singletons (N = 8,268,693) born in South Korea in 1997–2014. We first pooled all years and regressed birthweight on birth season while flexibly controlling for a large set of covariates. We then repeated the analysis by birth year and charted the trends in seasonality in birthweight. Results When we pooled all years, babies born in winter were the heaviest, while those born in summer the lightest; the difference in birthweight was about 11 g. When we analyzed the data by birth year, however, seasonality almost disappeared by the end of the period. Conclusions Whatever causes the seasonality has lost its influence in Korea. Replication studies can determine whether other countries exhibit the same patterns. Am. J. Hum. Biol. 28:767–773, 2016. © 2016Wiley Periodicals, Inc.