ABSTRACT BACKGROUND: Osteocalcin (OC) is the most common non-collagenous protein in bone matrix, which is synthesized only in bone tissue and by osteoblasts. The potential role of osteocalcin on glucose and fat metabolism has been previously reported. The aim of this study was to compare the serum OC level in pregnant women with and without gestational diabetes mellitus (GDM).METHODS: In the present case-control study, all pregnant women who were referred to a obstetrics and gynecology clinic in Sari, Iran, and met the inclusion criteria underwent an overall screening with a 75-g glucose tolerance test (GTT) at week 24 to 28 of gestation. The study was conducted between September 2018 and February 2019. Based on criteria, the pregnant women with confirmed GDM were matched with pregnant women without GDM in terms of baseline characteristics such as chronological age and BMI. The serum OC levels were also measured if vitamin D and calcium levels were normal. All data were analyzed using SPSS 21.RESULTS: The two groups with and without GDM had no significant difference in terms of age, BMI and OC level. There was no significant correlation between age and BMI with OC level in healthy pregnant women, respectively (P=0.49 and P=0.58). The correlation between BMI and age with OC level in GTT-positive pregnant women was 0.05 and -0.172, respectively, which was not significant (P=0.77 and P=0.36).CONCLUSION: According to the results of this study, there is no significant difference of serum OC levels in pregnant women with GDM compared to healthy pregnancy. Given that the levels of serum insulin or insulin resistance have not been assessed, these indices are recommended to be evaluated in future studies.KEYWORDS: Osteocalcin, Diabetes, Gestational, Vitamin D