Purpose of Investigation: To examine the relationship between maternal plasma progesterone along with corticotropin- releasing hormone (CRH) plasma levels and the progression of labor. Materials and Methods: Maternal serum CRH and progesterone were measured during the latent phase of labor, active labor, and 24 hours postpartum in women who went into spontaneous labor and delivered vaginally at term. Progesterone (P) levels in women delivered by an elective cesarean section at term were also measured as baseline. Results: Mean maternal plasma P was 18% higher in the active phase than in the latent phase of labor (p < 0.01), and declined significantly by 24 hours postpartum (p < 0.001). Mean level of serum CRH was 24% higher in the active phase than in the latent phase of labor (p < 0.01), and subsequently declined significantly by 24 hours postpartum (p < 0.001). Conclusions: As labor progresses, P and CRH increase and subsequently decrease precipitously in the immediate postpartal period. P levels tend to drop in women who are in early labor compared with non-laboring full-term women.