Introduction: Depression, anxiety and stress during pregnancy are associated with detrimental effects on maternal and child health. Writing is a cheap and non-invasive treatment that can improve mental health. This study was aimed to compare the effect of writing in two ways of expressive writing and daily activity writing on stress, anxiety, and depression in pregnant women.Methods: This clinical trial study was performed on 102 primiparous women with a gestational age of 25-28 weeks referred to the of South Health center of Tehran University of Medical Sciences. The samples were divided into two intervention groups and one control group by block randomization sampling method. Research instruments included demographic questionnaires, Beck Depression Inventory, and Depression Anxiety Stress Scale (Dass21). In the expressive writing group, women were asked to write their deepest feelings. In the daily activity writing group, daily activities were written without involving feelings and emotions. The control group received routine prenatal care. All three groups completed the DASS21 questionnaire before the intervention, four and eight weeks after the intervention. Data were analyzed using SPSS V21 software and ANOVA, Chi-square, and Fisher tests.Results: The variables of stress, anxiety, and depression were not statistically significant before the intervention in all 3 groups. The mean score of stress, anxiety, and depression in the two groups of expressive writing and daily activity writing was statistically significantly lower than the control group 4 weeks and 8 weeks after the intervention.Conclusion: According to the results of this study, writing can be recommended as a simple and low-cost method in reducing anxiety, stress, and depression in pregnant women during prenatal care.Trial registration: IRCT20170815035722N3. Registration date: 18-05-2019