Maternal age-related risk acceleration in hypertensive disorders of pregnancy: evidence for a primary change point around 29 years in a large retrospective cohort study

Hypertensive disorders of pregnancy (HDP) are major contributors to maternal and perinatal morbidity and mortality. Although maternal age is a well-recognized risk factor, its association with HDP is nonlinear, and the ages at which HDP risk begins to change more rapidly remain incompletely characterized. Current guideline-endorsed age thresholds of 35 and 40 years, widely used for advanced maternal age classification and risk stratification, were originally derived largely from fetal chromosomal risk and may not fully reflect the age-related risk profile of HDP. In this retrospective cohort study of 68,833 pregnancies delivered between 2013 and 2021, with partial 2022 data reserved for sensitivity analyses, maternal age was modeled as a continuous variable. Restricted cubic splines were applied to characterize nonlinear associations with HDP risk. A two-change-point segmented logistic regression model with grid search was used to identify data-driven age-related change points in HDP risk. Model performance was evaluated using the Akaike information criterion (AIC) and likelihood ratio tests. Diagnostic performance, including sensitivity and specificity, was compared across alternative age-based classification strategies, and sensitivity analyses were performed to assess robustness. Maternal age exhibited a nonlinear association with HDP risk, with a primary change point around 29 years and possible later acceleration around 36 years. The segmented model demonstrated a significantly better fit than the linear model ( P < 0.001) and showed better model fit than conventional age-classification strategies, including 35 years and 35/40 years (ΔAIC = 5.56). A threshold of ≥ 29 years was associated with higher sensitivity for case capture, whereas ≥ 36 years showed a different balance between sensitivity and specificity compared with ≥ 40 years, suggesting different trade-offs between case capture and specificity across age-based strategies. The risk of HDP begins to increase more rapidly around 29 years, with possible further acceleration around 36 years. This age-related pattern may refine the description of age-related HDP risk, but it should be considered alongside other clinical risk factors rather than used as a stand-alone criterion for prevention or management.