Objective:To disscuss the relationship between progesterone receptor(PR)expression in human placenta chorionic tissues and labor onset . Methods:PR protein levels and cellular localization were measured by immunohistochemistry and Western blot,respectively,in term pregnancy placenta tissue .Two clinical groups were selected:woman in labor(n = 15)and not in labor(n = 15). Results:PR immunostaining in placenta tissues of women not in labor was positive,and slightly positive in placenta tissues of women in labor. PR-A and PR-B levels decreased in labor group(P < 0.01) by Western blot. The PR-A to PR-B protein ratio also increased in labor group(P < 0.05). Conclusion:An increased PR-A to PR-B ratio may participate in the process of labor onset.