Study Protocol Patients meeting the inclusion criteria will have the study explained to them, including benefits risks and alternative therapy. Patients not desiring to participate in the study will receive our current regimen of preterm birth prevention (observation, risk factor assessment, steroids, progesterone, etc). If they accept inclusion into the study after the above explanation they will be randomized by the use of sequentially numbered, sealed opaque envelopes to either receive progesterone or a placebo injection on a weekly basis. Patients with risk factors for preterm birth (prior preterm birth) will be separately randomized from the multifetal gestations or cervical cerclage and the patients with preterm labor in the current pregnancy as well as preterm rupture of the membranes will also separately randomized (four groups total). Neither patients nor participants will know to which group (placebo vs. progesterone) they are assigned as the pharmacy will keep the randomization schedule and will dispense the two solutions in identical syringes for blinding purposes.