Effect of First Birth Interval Length on Preterm of Singleton Birth in Korea; 2010- 2013

Purpose: To compare the preterm birth (PTB) by length of first birth interval (LFBI) in singleton birth among primiparous women. Methods: Data from the 2010-2013 birth certification of Korea Statistics (718,103 singleton first births) was used. Extra-marital birth, teenage birth, non-hospital birth cases, pre-marital pregnancy (shotgun birth) cases, and missing information (year and month of marriage, gestational age) were excluded from the data. Odds ratio and 95% confidence intervals were calculated from multinomial logistic regression analyses to describe the associations between the LFBI and the risk of PTB. Results: The mean LFBI from marriage to first birth was 24.4 months. The frequency of LFBI was 29.7 percent in LFBI of 8-12 months, 38.1 percent in 13-24 months, 22.7 percent in 25-48 months, 3.8 percent in 49-60 months, and 5.7 percent in ≥61 months. The rate of PTB was increased with prolonged LFBI. The incidence of PTB by LFBI was 3.9 percent in the LFBI of 8-24 months, 4.3 percent in 25-48 months, and 5.8 percent in ≥49 months. The odds ratio of PTB for the LFBI of ≥49 months was 1.327 (95% confidence interval: 1.299-1.427), compared with the LFBI of 8-24 months. The odds ratio of PTB was 1.695 (1.598-1.798) in women aged 35 and older with the LFBI of ≥49 months, as compared to women aged under 34 years with the LFBI of ≥8-24 months. Conclusion: A prolonged LFBI was associated with the increased preterm birth risk.