A Multicenter, Placebo-controlled, Double-blind, Randomized, Parallel-group Study Assessing Efficacy of Progesterone Vaginal Tab for Prevention of Preterm Delivery

INTERVENTION: Subjects shall insert a progesterone tab (200mg) /day at bedtime into the vagina by themselves Subjects shall insert a placebo vaginal tab /day at bedtime into the vagina themselves CONDITION: Preterm delivery PRIMARY OUTCOME: The rate of pregnant women with the cervical length of 20 mm or more in 34 weeks of gestation SECONDARY OUTCOME: (1) Administration period of the progesterone vaginal tab; (2) The rate of cervical length shortening during the administration period of the progesterone vaginal tab; (3) Occurrence of obstetric concomitant diseases; (4) Occurrence of adverse events; (5) Number of weeks of gestation at the time of delivery; (6) Safety toward neonates INCLUSION CRITERIA: (1) Pregnant women between 16 weeks and 0 days and 23 weeks and 6 days in gestation when consent is provided and who visit the site as outpatients (2) Pregnant women whose cervical length is 20 mm <= and < 30 mm when consent is provided (3) Pregnant women who provide written consent for participation in this study