Objective: Gestational diabetes mellitus (GDM) is characterized with insulin resistance which is diagnosed during pregnancy. Although pregnancy is a diabetogenic state, not all women develop GDM. Genetic factors together with enviromental factors cause the maladaptation of maternal pancreas to this diabetogenic state and GDM develops. ADAMTS-9 is a recently recognized molecule whose genetic variants have risk of GDM. Decreased levels have already been shown in fetal membranes. Maternal serum levels of this protein have not been studied yet. We hypothesized that the alteration of ADAMTS-9 expression should cause changes in maternal serum levels which further could help to identify the disease and develop new treatment strategies.Materials and methods: This prospective case-control study is consisted of 27 pregnancies with GDM and 30 healthy singleton pregnancies matched for matenal age, gestational week, and maternal weight. GDM diagnosis was made with 2-h 75g oral glucose tolerance test. ADAMTS-9 levels were compared between groups.Results: ADAMTS levels were 3.620.33ng/dL (range: 3.04-4.23) in GDM group and 4.65 +/- 1.70ng/dL (range: 3.07-8.21) in control group (p<0.001). ADAMTS levels were not affected by maternal age, gestational age, and maternal weight.Conclusion: ADAMTS-9 levels were significantly lower in GDM pregnancies. This may help to understand the mechanism of GDM pathogenesis. In future, target treatments with ADAMTS proteins may help to improve the severity of diabetes pathogenesis.