BackgroundMothers suffering from common mental disorder (CMD), such as anxiety and depression may not be able to function properly, which could adversely affect the mother-infant bond and even result in increased infant morbidity and mortality. The purpose of this study was to assess the prevalence of CMD and its determinants among pregnant women in Southeast Ethiopia.MethodsData was collected from 743 pregnant women via interview-administered, standardised questionnaires during Dec–Jan 2017. The WHO Self-Reported Questionnaire (SRQ) was used to screen CMD. Multivariate logistic regression was conducted and ORs and 95% confidence intervals were calculated.ResultsThe prevalence of CMD during pregnancy was 35.8% (95% CI: 34–38%) and the main determinants of CMD were: illiteracy, presence of health risk, financial instability, physical or emotional abuse, having sexual intercourse without her willingness, family history of psychiatric illness and history of chronic medical illness.ConclusionCMD prevalence during pregnancy was high, indicating a need to regularly screen pregnant women for CMD and its determinants as part of routine obstetric care.