Patients diagnosed with arrested pre-term labor following tocolytics at 24-34 gestational weeks will be randomly allocated to receive either vaginal micronized progesterone 400 mg/day or no treatment.

Since progesterone derivatives are useful in preventing preterm labor in cases of risk factors or previous preterm labor, we hypothesize that they will also show efficacy in pregnancy prolongation in women whose preterm labor was arrested following tocolytic treatment.

Patients diagnosed with arrested pre-term labor following tocolytics at 24-34 gestational weeks will be randomly allocated to receive either vaginal micronized progesterone 400 mg/day or no treatment.

This study has the potential to find a treatment to prevent preterm labor and thus to reduce neonatal morbidity and mortality.

Inclusion Criteria:

1. 18 years of age
2. Tocolytic treatment between 24+0 and 34+0 weeks
3. Patient's consent to participate in this study
4. 24 hours after tocolytic initiation and up to 3 days after finishing the tocolytic treatment
5. Arrest of preterm labor

Exclusion Criteria

1. Contraindication to ongoing pregnancy including:

   1. Suspected amnionitis during testing for eligibility- evidence of active infection including temperature ≥ 38.0°C and uterine tenderness, foul-smelling vaginal discharge, maternal tachycardia of 120 beats per minute or greater, or sustained fetal tachycardia of 160 beats per minute or greater
   2. Evidence of significant placental abruption (contractions and significant bleeding from placental origin)
   3. Intrauterine fetal death diagnosed at the time of admission

2. Major fetal malformation

3. Known maternal allergy to progesterone

4. Current use of progesterone at the time of admission

5. Epilepsy

6. Breast cancer

7. PPROM (preterm premature rupture of membranes) during testing for eligibility

8. Age below 18 years

9. Known active liver disease (elevated liver enzymes at twice the upper normal limit according to medical history or blood test that were taking doring standard medical care)

10. History of deep vein thrombosis

11. Major active psychiatric disorders (major affective disorders and psychotic disorders)

12. Uncontrolled chronic hypertension

13. Heart failure

14. Chronic renal failure

15. Pre-gestational diabetes with known target organ damage

16. History of spontaneous preterm delivery

17. Previous tocolytic treatment during the current pregnancy