Description of pre-randomization procedures or process: Potential study subjects will be identified by Emergency Medical Service (EMS) personnel and Emergency Medicine attending physicians and residents in the ECC of GMH. EMS (ambulance) personnel will notify ECC personnel that they are bringing a blunt trauma patient with a Glasgow Coma Score (iGCS) of 12 or less. On arrival, the attending emergency physician on duty will conduct a primary and secondary trauma survey and note the patient's post initial resuscitation or i GCS. If the patient has a iGCS of 4 - 12 and meets criteria for presumptive eligibility, the ECC attending will page the investigator on-call (IOC) and ask Hospital Social Services to initiate aggressive efforts to contact the patient's family. To insure prompt administration of progesterone/placebo, a "door to needle time" of ≤ 2 hours has been established as our performance goal. To meet this deadline, a member of the research team will immediately respond to the ECC and determine if the patient meets the inclusion and exclusion criteria. Time from injury will be established as clearly as possible from the EMS report, police report or witness interview. Block randomization will be used to insure that moderate and severely injured patients of both genders and races (black and nonblack) have an equal chance of allocation to either treatment group. To accomplish this goal, the GMH pharmacy will be given block sets of randomly ordered packets with sequential study numbers for subjects with moderate TBI (presenting GCS 9-12) and severe TBI (presenting GCS 4-8). Inside each packet will be instructions to prepare an infusion of progesterone or a comparable volume of placebo. When the pharmacist receives a copy of the randomization request, he/she will open the next packet in sequence based on the patient's severity of brain injury (moderate versus severe), gender, and race. Following treatment group assignment, the pharmacist will record the patient's name, medical record number, dose, and time the medication was prepared in a confidential log for use by the Safety Committee.