Background: Pregnant women may experience constipation for the first time or their existing constipation symptoms increase in severity during pregnancy. Aim: To compare the effect of progesterone versus diet modification in the treatment of constipation during pregnancy. Subjects and Methods: Women aged ≥18 years with functional constipation according to the Rome III criteria from obstetrics outpatients’ clinic and midwife practices included in this study. Participants divided into two groups; control group managed with diet modifications and study group pregnant women with threatened miscarriage and advised to take vaginal progesterone ≥1 week. Participants completed a nonvalidated questionnaire created by the authors during the whole week before intake of progesterone or diet modifications and after treatment phase. Independent Student’s tâtest and Chiâsquare (ï£2) test were used for statistical analysis to compare between two studied groups. Primary outcome measures; change in defecation frequency. Results: Sensation of anorectal obstruction and sensation of incomplete evacuation were significantly less in Group B (progesterone therapy) compared to Group A (diet modification) (54% [154/281] and 62.98% [177/281] vs. 89.76% [614/684] and 91.08% [623/684], respectively) (P = 0.04 and 0.03, respectively). Straining during defecation and manual maneuvers to facilitate evacuation were significantly less in Group B compared to Group A (63.7% [179/281] and 19.9% [56/281] vs. 94.59% [647/684] and 86.54% [592/684], respectively) (P < 0.01 and 0.02, respectively). Episodes of abdominal pain and presence of reflux episodes were also significantly less in Group B compared to Group A (18.5% [52/281] and 17.43% [49/281] vs. 84.11% [589/684] and 75% [513/684], respectively) (P = 0.01 and 0.03, respectively). Conclusions: Estrogen, rather than progesterone, may be a detrimental factor of constipation during pregnancy via decreased bowel movement. Progesterone therapy seems to be effective in the treatment of functional constipation during pregnancy. A randomized placebo controlled trial is required to confirm the data of this study.