Preterm birth (PTB) is defined as birth before 37 completed weeks of gestation, which occurs in approximately 10% of all pregnancies. Prior PTB history and short cervical length (CL) are the most significant predictors of PTB. Prior PTB history can increase the risk of recurrence of PTB more than two-fold in the next pregnancy. A short CL of less than 25 mm as measured by ultrasound between 16 and 24 weeks of gestation has been shown to be the most reliable predictor of an increased risk of PTB. Progesterone is one of the few proven effective methods of preventing PTB in women with a previous history of spontaneous PTB and women with a short CL. Progestins are available in natural micronized or synthetic formulations for intramuscular or vaginal (tablet or gel) administration. Several studies have reported that 17 hydroxyprogesterone caproate injection can prevent recurrent PTB in women with a previous history of PTB. Vaginal micronized natural progesterone has also been shown to be effective in preventing PTB in women with previous PTB history or with a short CL. At present, we are performing a multi-center, randomized trial in Korea (a multicenter, randomized, open-label, investigator-initiated trial of vaginal compared with intramuscular progesterone for the prevention of PTB in high-risk pregnant women: VICTORIA protocol) to compare the efficacy between vaginal progesterone and intramuscular injection of progesterone in women with a previous preterm history or short CL. This study will provide important information to both obstetricians and patients on whether a vaginal or intramuscular regimen is better for prevention of a recurrent PTB.