The objective of this study was to analyze the distribution of gestational age by maternal age, and investigate the risk of early preterm ≤33 weeks), late preterm birth (34-36 weeks), early term (37-38 weeks), full term (39-40 weeks), late term (41 weeks) and post term (≥42 weeks) by analyzing 2008-2011 singleton birth of nulliparous women of certificated data (929,363 births) from Statistics Korea. Between 2008 and 2011, mean gestational age in nulliparous women decreased from 39.1 weeks to 39.0 weeks. The mean gestational age decreased with advanced age at birth. The slope of gestational age distribution in the nulliparous women aged ≤34 years was higher (kurtosis: 3.19) than that of the women aged 35 or older (2.43). Compared to women aged ≤34 years, women giving birth age at 35 and older were at increased risk for early preterm (odds ratio: 2.01, 95 percent confidence interval: 1.98-2.12), late preterm (1.48: 1.43-1.53), early term (1.46: 1.44-1.48) and post term (1.13: 1.03-1.25), and decreased risk for full term (0.70: 0.69-0.71) and late term (0.89: 0.87-0.91). An increasing use of induction labor and cesarean delivery were considered one of the factors for shifting gestational ages earlier.