Objective To explore the association between iron metabolism indexes and the risk of gestational diabetes mellitus(GDM)in pregnant women with advanced age.Methods A total of 292 pregnant women,whose age were≥35 years old and gave birth in Taicang First People's Hospital from April 2021 to April 2023,were retrospectively included and divided into GDM group and non-GDM group.The differences of iron metabolism indexes[serum ferritin(SF),serum iron(SI)and hemoglobin(Hb)]measured from the 20 to 24 weeks of gestation were compared between the two groups.Multivariable Logistic regression model was used to explore the association of SF,SI and Hb with GDM.Based on the data of single nucleotide polymorphism from IEU OpenGWAS(http://gwas.mr-cieu.ac.uk/)and FinnGen datasets,two samples Mendelian randomization analysis were conducted to explore the causal relationship between iron metabolism indexes and GDM by using the methods of Inverse Variance Weighted(IVW).Results In the maximally ad-justed multi-factor logistic models,the statistically significant association between SF measured from 20 to 24 weeks of gestation and the risk of GDM was found[odds ratio(95％confident interval)=1.02(1.01-1.04),P=0.001].The association between Hb and GDM was marginally significant[odds ratio(95％confident interval)=1.04(1.00-1.07),P=0.044],but no association between SI and GDM reached statistical significance.However,Mendelian randomization analysis showed there was no significant evidence for causal association between SF,Hb and GDM.Conclusion SF examined at 20 to 24 weeks of gestation could be used as a biomarker of GDM in the pregnant women with advanced age,but no evidence supported the causal association between SF and GDM.