BACKGROUND Intrauterine growth factor (IUGR) is one of the most important causes of neonatal mortality. The aim of this study was to evaluate the therapeutic effect of utrogestan on the treatment of IUGR and its complications. METHODS In this clinical trial, 66 pregnant women with idiopathic IUGR embryos were enrolled. Patients in the intervention group, in addition to receiving routine treatment of control group (high-protein diet, resting), took utrogestan capsules (100 mg) twice daily. The primary and secondary outcomes of the disease were recorded in a checklist. Data were analyzed using SPSS 18 using an independent t-test, Chi-square test, and Fisher's exact test. RESULTS In the intervention group, mean neonatal weight (P = 0.003), mean neonatal Apgar score (P = 0.001), and mean gestational age at birth (P = 0.001) were significantly higher than those in the control group. There was no neonatal death in the intervention group, whereas in the control group, four cases of neonatal death were observed (P = 0.03). In the majority of subjects in the intervention group, resistance index, and pulsatility index of the umbilical artery decreased (P = 0.002). The difference in abdominal circumference and gestational age in the intervention group decreased (P = 0.01). In the intervention group, the diastolic flow of the umbilical artery increased (P = 0.002). CONCLUSIONS Utrogestan was effective as an inexpensive and effective way to treat IUGR and improve pregnancy outcomes.