Objective: The aim of our study was to investigate the relationship between missed abortion and serum trimethylamine N-oxide (TMAO) levels. Methods: A total of 129 patients with 56 missed abortions and 73 healthy pregnancies were included in our study. Patients who had more than one pregnancy loss, had systemic disease (hypertension, diabetes, rheumatologic disease, hematologic disease, and so forth) and did not accept to participate in the study were excluded. Pregnant women who did not have a fetal heartbeat in the first 20th week of pregnancy were considered as missed abortion. Demographic characteristics of the patients were recorded. The serum TMAO levels of these patients were compared with the serum TMAO levels of healthy pregnant women with the same gestational week between the two groups. Results: The median (IQR) serum level of TMAO was significantly higher in woman with missed abortus compared to the healthy controls (201.5 [IQR, 129.75–345] vs 150 [IQR, 86.9–273], U = 1534, P = 0.015, rrb = 0.25 [95% CI: 0.05–0.43]). We observed a positive and significant relationship between serum TMAO levels and age of the patients (Spearman's rho = 0.272 [95% CI: 0.01–0.50], P = 0.043). However, no significant relationship was found between serum TMAO levels and BMI (Spearman's rho = 0.093 [95% CI: −0.18 to 0.35], P = 0.496). Conclusion: In our study, we found that the serum TMAO level was higher in patients with missed abortion compared to healthy pregnancies. Serum TMAO levels measured at early gestational weeks can provide information about the course of pregnancy.