Eligible patients will be informed and asked to enroll in the study at hospital admission. A transvaginal ultrasound examination will be performed to determine cervical length. If an eligible woman accepts to participate, patient will be randomized to one of the study arms assigned in a double blind basis. Patient will receive the medication (vaginal capsule of progesterone or placebo). The patient will administer herself one vaginal capsule in a daily basis since gestational age of 36 weeks and 6 days as the primary endpoint is to demonstrate that the use of a maintenance treatment with vaginal progesterone is able to reduce the incidence of preterm birth before 34.0 and 37.0 gestational weeks. After delivery, perinatal and neonatal data will be collected.

Inclusion Criteria:

• Pregnant women aged between 18 - 45 years
• Gestational age between 24.0 and before 33.6 gestational weeks
• Singleton pregnancy
• Patients admitted because of preterm labor who have been successfully treated of any tocolytic drugs (β-mimetics drugs, nifedipine, atosiban)
• Ultrasound cervical length at discharge < 25 mm
• Signed patient consent form (CI)

Exclusion Criteria:

• Known or suspected infection, premature rupture of membranes or any other pathological gravid maternal or fetal condition coexisting at hospital admission (Pre-eclampsia, RCIU) which may induce iatrogenic labor.
• Present or previous liver disease, present or previous cholestasis gravidarum, abnormal hepatic blood tests.
• With known allergy to progesterone or peanuts (excipient).
• Grade 2 (or upper) renal or liver laboratory abnormalities
• Treated from dependent-hormone malignant cancer (i.e., breast carcinoma...)
• With Diabetes mellitus or insulinized gestational diabetes
• Treated with heparin
• Drug abuse
• Inadequate treatment compliance