Secular Trends of Gestational Length Distribution in Korean Singleton and Twin Birth: 1997~99, 2011~13

ABSTRACT

PURPOSE: To compare the secular trends of gestational length distribution in singleton and twin birth by analyzing the birth certificated data of Statistics Korea. METHODS: The birth certificated data of Statistics Korea was used for this analysis (1997~99 1,850,236 births, 2011~13 1,325,301 births). The data of triplet birth cases, extra-marital birth, non-hospital birth cases, teenage birth, and gestational length information missing cases were excluded. Odds ratio and 95% confidence intervals were calculated from multinomial logistic regression analyses to describe the secular trend (1997~99 and 2011~13) of early preterm birth rate (≤33 weeks), late preterm birth rate (34~36 weeks), term birth rate (37~41 weeks), and post-term birth rate (≥42 weeks) adjusted for maternal age (20, 25, 30, 35, 40), birth order (1st=1, 2nd=2, ≧3rd=3), infantile gender (male=1, female=0), maternal occupation (none=1, yes=0), and education (≤middle=1, high=2, college/university=3). RESULTS: From phase I (1997~99) to phase II (2011~13), the gestational length distribution in singleton and twin births shifted the left and toward shorter gestational distribution, and mean gestational length decreased from 39.4 weeks to 38.7 weeks in singleton birth, and decreased from 36.9 weeks to 35.7 weeks in twin birth, During the period, the term birth rate of singleton birth was unchanged at 95 percent, but the term birth rate of twin birth was decreased from 65.7 percent to 46.1 percent. The odds ratio of early preterm birth, late preterm birth, term birth rate, and post-term birth rate in singleton birth for phase II were, respectively, 1.138 (95% confidence interval 1.106~1.171), 1.556 (1.532~1.581), 1.094 (1.081~1.107), and 0.113 (0.109~0.118), compared with rate of each gestational length for phase I. The odds ratio of each gestational length in twin birth of phase II, comparing phase I, were 1.495 (1.405~1.590) for early preterm birth, 2.110 (2.029~2.194) for late preterm birth, 0.444 (0.428~0.461) for full term birth, and 0.055 (0.026-0.117) for post-term birth. CONCLUSION: The risk of early preterm birth, late preterm birth between phase I & II increased higher in twin birth than that of singleton birth. The rise in early and late preterm birth, and decrease in mean gestational length in singleton and twin birth during the period. There was a need to more research in this area to understand the contributing factors to gestational length.