The objective of this study was to analyze the risk of preterm birth (PTB) and low birth weight (LBW) in extramarital birth by analyzing 2008-2012 birth certificated data (2,328,719 births) from Korea Statistics. Odds ratio and 95% confidence intervals (95% CI) were calculated from logistic regression analyses to describe the associations between PTB & LBW and extramarital birth adjusted for maternal age, maternal occupation & education, infantile sex, birth order and number of child birth (singleton & multiple birth). The rate of extramarital birth was 1.8 percent and 2.1 percent in 2010-11. The incidence of LBW was 8.1 percent in extramarital birth and 5.0 percent in marital birth. The incidence of PTB in extramarital birth were 8.2 percent and 5.8 percent in marital birth. Compared with marital birth, the odds ratio (OR) for PTB were 1.48 (95% CI: 1.43-1.54) for extramarital birth. Risk of LBW was higher in extramarital birth (OR: 1.70, 95% CI: 1.64-1.76) than that of marital birth. Among mothers younger than 20 years, the odds ratio of PTB among extramarital birth, relative to married birth was 1.69 (1.49-1.91). Among unmarried mothers, those at a higher risk of LBW was aged 20-29 years (1.69: 1.59-1.79). Maternal unmarried status was associated with increased risk of PTB and LBW.