Dose-response associations of maternal age with pregnancy complications and multimorbidity among nulliparas and multiparas: A multicentric retrospective cohort study in southern China.

Background: Advanced maternal age is becoming an increasingly common issue worldwide, presenting substantial health risks to pregnant women. However, dose-response associations of maternal age with a comprehensive range of pregnancy complications and their multimorbidity remain unclear. Methods: We conducted a retrospective cohort study using data from China's National Maternal Near Miss Surveillance System for 2017-2018, including 18 hospitals in southern China. We included 135 274 pregnant women aged 15-54 years with a singleton birth. We used multivariable logistic regression and restricted cubic spline to examine dose-response associations between maternal age and various pregnancy complications, as well as multimorbidity. We employed the Apriori algorithm to mine the association rules among pregnancy complications and identify frequent multimorbidity patterns. Results: We found three distinct patterns of associations between maternal age and specific pregnancy complications. In relation to increasing maternal age, gestational diabetes mellitus, preeclampsia, and gestational hypertension showed nonlinear increasing trends for both nulliparas and multiparas, as did multimorbidity in nulliparas. Conversely, we observed linear increasing trends for placental previa in both nulliparas and multiparas, placental abruption in nulliparas, and multimorbidity in multiparas. Infection and severe anaemia had an approximate J-shaped curve among nulliparas, while postpartum haemorrhage exhibited a similar curve in both nulliparas and multiparas. Advanced maternal age was linked to an elevated risk of multimorbidity during pregnancy or postpartum period, exhibiting more complicated patterns. The most common multimorbidity patterns in this age group were "preeclampsia + gestational diabetes mellitus", "gestational hypertension + gestational diabetes mellitus", "infection + gestational diabetes mellitus", and "placental previa + gestational diabetes mellitus". Conclusions: Maternal age was associated with pregnancy complications and multimorbidity in three broad dose-response manners, including approximate J-shaped curves, as well as nonlinear and linear increasing trends, depending on the specific outcome and parity, which may suggest different underlying biological mechanisms. Women with advanced maternal age had a higher risk and more complicated patterns of multimorbidity during pregnancy or postpartum, suggesting that this group should be targeted for more intensive health care.