To determine the infusion rate needed to achieve a SSSPC of progesterone of 450 +/- 100nmol/L, we will conduct a dose-escalation study. The first 12 subjects randomized to progesterone will receive a loading dose of 0.714 mg/kg over 60 minutes followed by a continuous infusion of 0.50 mg/kg/hr for three days. Because study personnel will be blinded to group assignment, an unblinded pharmacologist will analyze the SSSPC data from these 12 patients. If the target concentration of 450 +/- 100nmol/L is not achieved in 90% of these subjects, the infusion rate will be modified and applied to the next ten subjects randomized to progesterone therapy. This will be performed a third time if necessary. If pharmacokinetic variability precludes the achievement of the target concentration in 90% of subjects using a fixed infusion, we will adopt an individualized dosing strategy based on real-time serum concentration data. If this is required, adjustments will be made to the placebo infusions as well to maintain blinding of the study investigators. The total dose given to any patient over the three day dosing period should not exceed 7.5 grams.