Vaginal PROgesterone as Maintenance Treatment After an epISode of prEterm Labor (PROMISE Study)

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