Flow chart of the study design. In all, 597 patients with RIF were recruited and divided into two groups based on whether or not they agreed to undergo the matrix metalloproteinase (MMP) test. Patients with MMP scores ≥5 were deemed to have a positive response (Group P) and were treated with a corticosteroid and an antibiotic for 2 weeks. Patients with low MMP scores (<5) were determined as negative (Group N). Patients in Group P were given a second MMP test after the treatment and were divided into two subgroups: Group Pr (patients who had again positive results and were considered as resistant to the treatment) and Group Pe (patients in whom the treatment was effective). Patients who did not undergo an MMP test comprised the control group (Group C). Patients in all groups underwent another embryo transfer procedure and the outcomes were compared among the four groups.