Objective: To study the prostanoid profile of the endometria of patients with recurrent implantation failure (RIF), unexplained infertility (UIF), and recurrent miscarriages (RM), and to compare them with the endometria of healthy fertile controls. Design: Prospective cohort study. Setting: University hospital. Patient(s): Fifteen patients with RIF, 18 patients with UIF, 16 patients with RM, and 23 fertile controls were recruited. Intervention(s): Endometrial samples were taken during the window of implantation. After tissue homogenization and extraction, analysis with ultra-performance liquid chromatography diode array detector electrospray ionisation tandemmass spectrometrywas performed. Main Outcome Measures: Concentrations of prostaglandin (PG) D1, PGE1, PGF1 alpha, 6-ketoPGF1 alpha GD2, PGE2, PGF2 alpha, 15-deoxy-Delta 12,14-PGJ2, PGD3, PGE3, PGF3 alpha, thromboxane B2, 13,14-dihydro-PGE1, 13,14-dihydro-PGF1 alpha, 13,14-dihydro-PGF2 alpha, 13,14dihydro-15-keto-PGE1, 13,14-dihydro-15-keto-PGE2, and 13,14-dihydro-15-keto-PGF2 alpha were assessed. Result(s): Comparison of the endometria of patients with UIF and the controls showed no statistically significant differences. When the endometria of patients with RIF were compared with the controls, thromboxane B2 (TXB2) was found significantly higher (843.1 pg/mg vs. 133.5 pg/mg). When the endometria of patients with RM were compared with controls, 13,14-dihydro-15-keto PGF2 alpha and TXB2 were found significantly higher (3907.30 pg/mg vs. 17.80 pg/mg and 858.7 pg/mg vs. 133.5 pg/mg respectively). Conclusion(s): We identified increased endometrial presence of TXB2 in patients with RM and RIF, and 13,14-dihydro-15-keto PGF2 alpha in patients with RM. Although common ground is observed for RM and RIF, prostanoids, on the other hand, might make their own contribution to endometrial receptivity as important as genes and proteins. Attempts to normalize the prostaglandin profile of the endometrium via enzymatic activity can open new therapeutic options. (C) 2019 by American Society for Reproductive Medicine.