The impact of maternal age and body mass index on hypertensive disorders of pregnancy: moving beyond the cut-off effect

Background: Maternal age (MA) and body mass index (BMI) are known risk factors for hypertensive disorders of pregnancy (HDP). Different threshold values are used to calculate preeclampsia risk scores, but the appropriateness of a cut point model has not been extensively evaluated. This is because the effects of both MA and BMI occur continuously. We aimed to investigate the relationship between MA, BMI, and HDP, respecting the continuous nature of the two independent variables. Materials and Methods: We retrospectively selected all nulliparous women with singleton pregnancies who delivered after 22 gestational weeks from January 2005 to December 2019 (25,165 women). We used univariate and multi- variable logistic regression analyses implementing linear, quadratic, cubic, and penalized splines functions to test, investigate, and describe the relationship between continuous BMI, continuous MA, and risk of HDP. Results: MA, BMI, and infertility treatments are independently associated with the risk of developing all HDP in nulliparous women. The impacts of MA and BMI on the risk of developing HDP occur gradually, and penalized splines functions resulted in the best model to describe these associations with a different model for each HDP. No interaction factors were retained between MA, BMI, and infertility treatments. Conclusions: Given the relevance of HDP on maternal mortality, an accurate identification of women at increased risk of developing the disease is crucial. In an era where technology is rapidly advancing, new models for customized risk assessment of HDP, considering the continuous nature of MA and pregestational BMI, must be implemented.