Statistical design and sample size calculations: Our data analysis strategy is designed to accomplish our three primary and one secondary goals: 1) achieve and maintain a predetermined steady state concentration of progesterone in 90% of our study patients, 2) confirm the safety of the study drug, 3) test the hypothesis that administration of progesterone within 6 hours of TBI improves clinical and neurological outcomes, and the secondary; determine if the benefit of IV progesterone is equally evident across a range of clinically relevant treatment subgroups. In the process of achieving these goals, we will quantify our ability to identify and enroll TBI patients, administer the study drug within strict time restraints, and adhere to the study protocol.