PURPOSE: The objective of the study was to compare contribution of maternal age to preterm birth (PTB) rates between 1997~99 and 2012~14. METHODS: We used 1997~99 (1,872,720 births) and 2012~14 (1,280,348 births) singleton birth certificated data of Korea Statistics excluding multiple birth and extra-marital birth cases. We decomposed the contributions of age-specific PTB rates and maternal age distribution (Kitagawa's decomposition method) to overall PTB rates during the period. Odds ratio (OR) and 95% confidence intervals were calculated from logistic regression to describe the secular trend of PTB rate by birth year and maternal age. RESULTS: The incidence of PTB increased 1.5 times, from 3.0 percent to 4.6 percent, during 1997~2014. After adjustment by logistic regression for infantile sex, parity and maternal age, the odds ratio of PTB in birth year of 2012~14 was 1.38 (95% confidence interval: 1.36~1.39), compared with incidence of PTB in 1997~99. During the period, PTB rates increased 1.79 times in women aged 20~24 years and 25~29 years (OR: 1.56), whereas rates remained stable in women aged 35 years and older (OR: range from 1.08 to 1.13). 87.5% of the overall increase in the PTB rates was attributable to the increase in the proportion of women aged 30~34 years, but these age group accounted for only a small portion of the increase in PTB rates (OR: 1.27). CONCLUSION: The remainder of the total increment in the PTB rates during the period was explained by increase the proportion of women aged 30~34 years. There was a need to close attention in this area to understand the contributing factors to the secular trend of PTB rates.
Female ; Humans ; Incidence* ; Korea* ; Logistic Models ; Maternal Age* ; Multiple Birth Offspring ; Odds Ratio ; Parity ; Parturition* ; Premature Birth*

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.
Birth Order ; Birth Rate ; Education ; Humans ; Korea ; Logistic Models ; Maternal Age ; Occupations ; Odds Ratio ; Parturition* ; Premature Birth ; Term Birth ; Triplets ; Twins*

PURPOSE: To compare the incidence of twin births with the length of first birth interval (LFBI: from marriage to first birth) among primiparous women. METHODS: The birth certificated data of Korea Statistics from 2010 to 2015 were used for this analysis. There were 1,356,204 births of primiparous women from total birth cases (2,736,296 births). The data involving multiparous women, teenage birth, extra-marital birth, and triplet birth cases were excluded from all analyses. Odds ratio and 95 percent confidence intervals were calculated from logistic regression to examine the risk of twin birth by LFBI adjusted for year of birth, maternal age, paternal age, age difference between couples, and maternal education and occupation. RESULTS: The frequency of LFBI was 41.5 percent in ≤12 months, 30.7 percent in 13~24 months, 13.1 percent in 25~36 months, and 2.1 percent in LFBI of ≥85 months. The mean LFBI was 21.5 months. The incidence of twin birth increased with prolonged LFBI. The twin birth rates per LFBI were 0.8 percent, 3.1 percent, 7.9 percent, and 11.0 percent in LFBI of ≤12 months, 25~36 months, 49~60 months, and 73~84 months, respectively. Odds ratio of twin births rate by LFBI were 1.510 (95% confidence interval: 1.449~1.574) for the LFBI of 13~24 months, 9.839 (9.390~10.309) for 49~60 months, and 13.244 (12.458~14.080) for ≥73~84 months, each compared with LFBI of ≤12 months. Odds ratio of twin birth rate in LFBI of ≤72 months was higher in women aged 35 and older, as compared to women aged ≤34 years. Odds ratio of twin birth rate by maternal age (≤34 years versus ≥35 years) reversed in LFBI of ≥73 months. CONCLUSION: The risk of twin birth increased significantly with prolonged LFBI. There is a need to understand the factors (fertility therapy and etc.) to increased risk of twin birth in prolonged LFBI.
Birth Intervals* ; Birth Order* ; Birth Rate ; Education ; Family Characteristics ; Female ; Humans ; Incidence* ; Korea* ; Logistic Models ; Marriage ; Maternal Age ; Occupations ; Odds Ratio ; Parturition* ; Paternal Age ; Triplets ; Twins*

PURPOSE: To figure out the contribution of maternal age distribution to the preterm birth (PTB) rate of multiple births between 1997~98 and 2014~15. METHODS: Multiple birth certificate data of Korea Statistics were used for this analysis. There were 18,557 births in 1997~98 and 30,992 births in 2014~15. Kitagawa's decomposition method was used to examine the contribution of age-specific PTB rate and maternal age distribution of multiple births to overall increment of PTB rate in multiple births between 1997~98 and 2014~15. RESULTS: PTB rate of multiple births increased from 32.40 percent to 58.22 percent (odds ratio: 1.80, 95% confidence interval: 1.76~1.84) during 1997-2015. PTB rate of multiple births greatly increased for women aged 25~29 years (odds ratio: 2.09) during the same period. The rates increased 1.88 times for women aged ≤24 years, followed by women aged 30~34 years (OR: 1.65), women aged 35~39 years (1.54), and women aged ≥ 40 years (1.36). Most (78.7%) of the overall increment in PTB rate of multiple births was attributable to the increase in the dimension of women aged 30~34 years, and 49.9 percent for women aged 35~39 years. CONCLUSION: The total increment in the PTB rate of multiple births was explained by increase the proportion and the PTB rate of women aged 30~39 years. More research is needed to comprehend the contributing age factors to PTB rate of multiple births.
Age Factors ; Female ; Humans ; Incidence* ; Korea ; Maternal Age* ; Methods ; Multiple Birth Offspring* ; Parturition ; Premature Birth*

PURPOSE: To compare the relationship between intertwin birth weight discordance and maternal age at birth. METHODS: There were 51,726 pairs of twins born from 2007 to 2014 according to the birth certificate data of Korea Statistics (3,701,806 births). The data were excluded due to extra marital births, home births, and teenage births. Birth weight discordance rate (%) was calculated according to the following formula: (larger birth weight - smaller birth weight)/(birth weight of the larger twin) ×100. The odds ratio of intertwin birth weight discordance rate (≥25%) in maternal age at birth was calculated by logistic regression adjusted by the year of birth, gestational age, maternal education, infantile gender and paternal age. RESULTS: The mean birth weight of the heavier twin and that of the lighter twin were 2.536 kg and 2.254 kg, respectively. The mean birth weight difference of intertwins was 0.282 kg. The mean birth weight discordance rate by maternal age was 10.5 percent for maternal ages between 20 and 29 years, 11.0 percent for between 30 and 34 years, and 11.4 percent for 35 years and older. The frequency of birth weight discordance level of ≥25 percent increased as the maternal age increased. The rate of birth weight discordance (≥25%) was 7.7 percent for maternal ages between 20 and 29 years, 8.3 percent for between 30 and 34 years, and 8.7 percent in for 35 years and older. Compared with women from 20 to 29 years of age, the odds ratio of ≥25 percent discordance was 1.094 (95% confidence interval: 1.005~1.190) for 30 to 34 years, and 1.164 (1.401~1.301) for 35 years and older. The odds ratio of ≥25 percent discordance for the different-sexed twins was higher than that of the same-sexed twins in overall maternal age groups of 20 to 29 years, 30 to 34 years, and 35 years and older. CONCLUSION: The risk of birth weight discordance level of ≥25 percent was associated with the older maternal age. More research is required to understand the risk factors of intertwin birth weight discordance for older women.
Birth Certificates ; Birth Weight* ; Education ; Female ; Gestational Age ; Humans ; Korea* ; Logistic Models ; Maternal Age* ; Odds Ratio ; Parturition* ; Paternal Age ; Risk Factors ; Twins

PURPOSE: To determine the secular trend of the multiple birth rate in Korea from 1981 to 2017. METHODS: This study used birth certificate data covering the years 1981–2017 (20,948,901 births), provided by Statistics Korea. The impact of assisted reproductive technologies (ART) on multiple birth was analyzed by examining data prior to and after the introduction of ART and the national support program for infertile couples in South Korea. The odds ratio (OR) and 95% confidence intervals were calculated to describe the secular trend in the multiple birth rate per 100 births. RESULTS: During this period, the multiple birth rate per 100 births increased by 277 percent from 1.031 to 3.891, the twin birth rate increased from 1.013 to 3.807 (275%), and the triplet birth rate increased from 0.018 to 0.084 (363%). The secular trend of the multiple birth rate remained in the 1.000 level during 1981–1991, but has been rising steadily since 1992. The average increment of the multiple birth rate was 3.9% annually from 1981 through 2017 (peaking at more than 10 percent during 2006–2007). Prior to the introduction of ART in Korea (1981–1984), the multiple birth rate was 1.007, but after its introduction, the multiple birth rate was 1.005 in 1985–1989 (OR, 0.998; 95% confidence interval, 0.982–1.014), 1.084 in 1990–1994 (1.007; 1.060–1.094), 1.891 in 2000–2004 (1.894; 1.866–1.922), 3.127 in 2010–2014 (3.173; 3.129–3.217), and 3.811 in 2015–2017 (3.893; 3.835–3.952). CONCLUSION: Over the past three decades, multiple births have risen dramatically in Korea, primarily due to the increasingly widespread use of fertility therapies, and the delayed age of childbearing. There is a need for more research to understand the factors contributing to multiple births, and the national birth registration systems ought to be reformed to monitor whether multiple births originated spontaneously or from assisted-conception.
Birth Certificates ; Birth Rate ; Family Characteristics ; Fertility ; Humans ; Korea ; Multiple Birth Offspring ; Odds Ratio ; Parturition ; Reproductive Techniques, Assisted ; Triplets ; Twins

PURPOSE: To investigate the incidence of intra-twin birth weight discordance and its association with infantile gender and birth order. METHODS: We used the data of birth from 2008 to 2013 of Korea Statistics (38,140 pairs of twins). Adjusted logistic regression analyses were performed to describe the birth weight discordance (> or =25%) according to infantile gender and birth order (twin A: 1st, twin B: 2nd). Birth weight discordance was calculated as 100 x (birth weight difference/birth weight of the heavier twin). RESULTS: Twin A (mean 2.431 kg) was heavier than twin B (mean 2.359 kg), and the weight difference was 0.281 kg on average. Overall incidence of birth weight discordance was 8.3 percent. The incidence of discordance was 8.8 percent among unlike-sexed pairs and 8.0 percent among like-sexed pairs. Twins with birth weight A> or =B (10.2 percent) showed higher incidence of discordance than twins with birth weight A or =B) as compared to like-sexed twins (female-female) with birth weight (A Birth Order* ; Birth Weight* ; Body Weight ; Humans ; Incidence ; Korea* ; Logistic Models ; Odds Ratio ; Parturition* ; Twins