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.