Background: When individuals are aware of the appropriate ethical practice, but lack the ability to do it, they will suffer from moral distress. Moral distress is a frequent phenomenon in clinical practice which can have different effects on the performance of physicians, nurses, and midwives, and therefore patients and health care systems. Research objective: The present study aimed to determine the severity and frequency of moral distress in midwives working in birth centers. Research design: This study is a descriptive cross-sectional research. Researcher-made questionnaire was used to gather data. Participants and research context: A total of 180 midwives working in the labor ward of the public birth centers affiliated to Shahid Beheshti University of Medical Sciences were included to the study by census. Ethical considerations: Official permission for data collecting was obtained from the directors of the birth centers affiliated to Shahid Beheshti University of Medical Sciences. Then, after explaining the objectives of the study and assuring the confidentially of information, verbal consent of the participants was obtained. Findings: The total mean ± standard deviation of the severity and frequency of moral distress were 3.85 ± 0.75 and 3.03 ± 0.48, respectively. The highest severity and the lowest frequency of moral distress were obtained for the assistance for abortion and the lowest severity of moral distress was related to the organizational domain. However, the highest frequency of moral distress was related to futile care field. The mean of moral distress severity in the midwives with associate degree was significantly lower than other levels of education. Also, there was a significant relationship between age and moral distress frequency (p = 0.010). Discussion: The midwives’ moral distress was relatively high as expected. This finding is consistent with the results of similar studies in intensive care unit nurses. Conclusion: After identifying the level and most important factors of moral distress among midwives, the next step is empower them to prevent moral distress, in particular efforts to change structures.