ABSTRACT Background: Miscarriage is interruption or termination of pregnancy before 20 weeks based upon the day of the last normal menses or it is the expulsion of the product of conception before the age of medicolegal viability which is 26th weeks of gestation in Egypt or fetal weight < 500gm or fetal length ≤ 25 according to WHO definition. Objective: This study was performed to compare the effect of vaginal micronized progesterone and oral dydrogesterone on utero-placental circulation in threatened miscarriage. Patients and Methods: A prospective clinical study in Zagazig University Hospital during the period from December 2018 to August 2019. Included fifty pregnant cases complaint of first trimester vaginal bleeding and diagnosed as threatened miscarriage. They were classified into two groups on a randomized basis, the first group received micronized vaginal progesterone (400 microgram daily) while the second one received dydrogesterone orally (40 mg at once then 10 mg every 8 hours). Both groups were followed up every two weeks for three visits by serial transvaginal Doppler ultrasound measurement of pulsatility index, resistance index, and systolic/diastolic (S/D) ratio of the spiral arteries, uterine arteries & intrachorionic area. Results: The study demonstrated that vaginal progesterone administration, contrary to oral dydrogesterone treatment, decreased spiral artery pulsatility and resistive indices. No major changes in the uterine artery blood flow impedance were observed, only S/D ratio significantly decreased in the dydrogesterone group. Conclusion: It could be concluded that vaginally administrated progesterone was nearly as equally effective as oral dydrogesterone in prevention of miscarriage in pregnant women with threatened abortion with different effects on the uteroplacental circulation.