The investigators study objective is to investigate the effectiveness of vaginal micronized progesterone in prolonging pregnancy after an episode of preterm labor which responded to tocolytic treatment. Furthermore, the investigators aim is to study the mechanisms of actions of progesterone supplementation by examining its effect on the two components of parturition: cervical ripening and myometrial contractility. The investigators will utilize transvaginal ultrasound to assess the changes in the cervical tissue, and non-invasive trans-abdominal uterine EMG to assess the uterine muscle activity. The investigators will also look at the effect of progesterone on contraction frequency by tocodynamometer (TOCO), though EMG is expected to provide much more information.

Inclusion Criteria:

• Patients with singleton pregnancies at gestational ages 25+0 to 33+6 weeks after an episode of preterm labor, with cervical length ≤25mm:
• Patients presenting with signs and symptoms of preterm labor will be admitted and given the standard treatment for preterm labor, i.e. tocolysis for 48 hours, and antenatal steroids.
• Those who will remain undelivered for 48 hours and will be found to have a cervical length of 25mm or less will be offered entrance into the study.
• Preterm labor will be defined as at least 4 contractions in 30 minutes with cervical change assessed by digital cervical examination.

Exclusion Criteria:

• Patients with preterm premature rupture of membranes (PPROM) will be excluded from the study, due to the possibility of ascending infection, as will those with suspected chorioamnionitis.
• We will not include multiple pregnancies.
• Patients with medical conditions that contraindicate tocolysis, such as non-reassuring fetal heart tracings, will also be excluded.