Preterm birth remains a major clinical problem. Prematurity is not only a major cause of perinatal mortality but also leads to greater risk for short and long term complications including disability and impediments in growth and mental development. To compare vaginal with intramuscular progesterone administration to prevent preterm labor and to detect the effect of both on the uterine and foetal circulations. STUDY DESIGN: comparative interventional study. POPULATION: 100 pregnant women attending NRSMCH Kolkata, Obstetrics emergency with threatened preterm labour before 34 weeks of gestation. STUDY PERIOD: one year. METHODS: Gestational age was confirmed clinically and by USG of early weeks of gestation. Efficacy and tolerability of progesterone in two routes i.e., intramuscular and vaginal for prevention of threatened preterm labour were compared. OUTCOME: Ante partum, intrapartum and perinatal outcomes were compared between two groups by statistical analysis of data's using chi-square test and student-t test. CONCLUSION: Vaginal progesterone was as effective as intramuscular progesterone in reducing preterm birth with fewer side effects in favor of vaginal route.