Nationwide Population-Based Cohort Study of Advanced Maternal Age and Obstetric Outcomes in Korean Women: Retrospective Cohort Study.

Abstract

Background: The proportion of mothers older than 35 years is increasing; however, research examining the age at which pregnancy is considered high risk across different races and countries is limited. Therefore, this study aimed to determine the cut-off age at which adverse obstetric outcomes significantly increase by investigating age-specific incidence rates of major adverse obstetric outcomes in Republic of Korea (South Korea).

Methods: We analyzed the Korean National Health Insurance claims database between 2015 and 2021, focusing on women with obstetric complications classified using the International Classification of Diseases, Tenth Edition. The maternal cut-off age was defined as the age at which the outcome was most predictive in receiver operating characteristic curve analysis. Multivariate logistic regression was used to estimate the adjusted odds ratio (OR) and 95% confidence interval (CI) for the association between maternal age and adverse obstetric outcomes.

Results: The study included 1,862,393 women with singleton pregnancies. Of these, 33.67% were aged ≥ 35 years and 5.07% were aged ≥ 40 years. The incidence of adverse obstetric outcomes increased linearly and positively with maternal age. The maternal cut-off age, determined using receiver operating characteristics analysis, was 35 years for pregnancy-induced hypertension (PIH), 33 for gestational diabetes mellitus (GDM), 33 for placenta previa, 32 for placental abruption, 34 for preterm delivery, and 34 for low birth weight (LBW). In multivariate analyses, adjusted OR revealed adverse obstetric outcomes: PIH (OR, 1.29; 95% CI, 1.27-1.32), GDM (OR, 1.73; 95% CI, 1.72-1.75), placenta previa (OR, 1.81; 95% CI, 1.78-1.85), placental abruption (OR, 1.30; 95% CI, 1.24-1.36), preterm delivery (OR, 1.32; 95% CI, 1.29-1.34), and LBW (OR, 1.33; 95% CI, 1.31-1.35). For the composite outcome, maternal cut-off age was found to be 33 years (OR, 1.57; 95% CI, 1.56-1.59), with an area under the curve of 0.573.

Conclusion: This nationwide study defined the cut-off age for advanced maternal age in Korean women, showing that obstetric risks begin to increase in the early 30s. These population-specific thresholds may help guide clinical practice and improve maternal and perinatal outcomes.