OBJECTIVE: The purpose of this study was to investigate the temporal changes in immunoreactive cystatin A and the enzymatic activity of cathepsins B, H, L, and S in human cervicovaginal fluid (CVF) in late pregnancy and spontaneous labor. STUDY DESIGN: CVF was collected weekly (n = 95 women) from 36 weeks gestation until spontaneous term labor. Cystatin A was quantified using enzyme-linked immunosorbent assay. The enzyme activity of cathepsins B, H, L, and S was measured with fluorometric enzyme assay kits. RESULTS: Cystatin A significantly decreased towards (P = .016, 2-way analysis of variance) and during labor (P .001, 2-way analysis of variance). Enzymatic activity of cathepsins B, H, and S did not change with labor onset (P = .452, P = .703, P = .411, respectively, 2-way analysis of variance). CONCLUSION: In late gestation, CVF-decreased expression of the cysteine protease inhibitor, cystatin A, is associated with labor. Although the role and contribution of cystatin A to increased extracellular matrix remodeling has yet to be elucidated, the data that were obtained are consistent with this hypothesis.