Secular Trends in Sex Ratio at Birth by Birth Order in South Korea, 1981–2017
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Objectives: This study analyzed secular trends in sex ratio at birth in Korea by birth order using birth records from Statistics Korea between 1981 and 2017. Methods: A total of 21,685,402 birth records were used and the sex ratio was measured as the number of males per 100 females. Sex order was categorized into the first, second, and third and higher. Births of unclassified sex order (n = 47,445) were excluded from the analysis. We conducted logistic regression analyses to test if there are significant changes in sex ratio at birth for all other periods, using the period 1981-1984 as the reference group, and to test if there are significant changes for the higher order births, using the first order births as the reference group. Odds ratio and 95% confidence interval were calculated for each period. Results: Since having peaked at 116.5 in 1990, the ratio for total births gradually declined to reach the lowest point of 105.0 in 2016. The sex ratio increased with rising birth order. A baby born for the second and the third and higher birth order was 1.07 (95% confidence interval, 95% CI: 1.06-1.08) and 1.86 (95% CI: 1.85-1.88), respectively, times more likely to be male than a first-born baby for period 1990-1994. Conclusions: Having peaked at 209.7 in 1993, the sex ratio for the third and higher order births reverted to the naturally occurring level of 105.5 in 2015. Despite the return, the abnormally high sex ratio for the third and higher order births persisted until the early 2010s.
Abstract
Objectives: This study analyzed secular trends in sex ratio at birth in Korea by birth order using birth records from Statistics Korea between 1981 and 2017. Methods: A total of 21,685,402 birth records were used and the sex ratio was measured as the number of males per 100 females. Sex order was categorized into the first, second, and third and higher. Births of unclassified sex order (n = 47,445) were excluded from the analysis. We conducted logistic regression analyses to test if there are significant changes in sex ratio at birth for all other periods, using the period 1981-1984 as the reference group, and to test if there are significant changes for the higher order births, using the first order births as the reference group. Odds ratio and 95% confidence interval were calculated for each period. Results: Since having peaked at 116.5 in 1990, the ratio for total births gradually declined to reach the lowest point of 105.0 in 2016. The sex ratio increased with rising birth order. A baby born for the second and the third and higher birth order was 1.07 (95% confidence interval, 95% CI: 1.06-1.08) and 1.86 (95% CI: 1.85-1.88), respectively, times more likely to be male than a first-born baby for period 1990-1994. Conclusions: Having peaked at 209.7 in 1993, the sex ratio for the third and higher order births reverted to the naturally occurring level of 105.5 in 2015. Despite the return, the abnormally high sex ratio for the third and higher order births persisted until the early 2010s.
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