Wu J, et al. - Women undergoing controlled ovarian stimulation for IVF or oocyte cryopreservation using conventional controlled ovarian stimulation (COS) vs minimal stimulation with clomiphene citrate (MS-COS) were compared regarding the histologic profiles of the endometrium. At time of oocyte retrieval (35h after hCG), collection of endometrial tissue was done from patients undergoing either COS (n = 8) or MS-COS (clomiphene citrate (CC) + hMG, n = 8) protocols. Two gynecologic pathologists blinded to treatment performed histopathology assessment of H&E stained sections (n = 3 in each group) independently using Noyes criteria. They noted dramatic impairment in endometrial development, most likely due to clomiphene citrate, despite significant increases in E2 during MS-IVF (875 vs ∼200 pg/ml for natural cycles). Thinning of the endometrial stripe was identified in sonography, and major compromise of epithelial growth, differentiation, and density with relative increases in endometrial stroma were identified in histology. On the other hand, in COS, they noted supraphysiologic estradiol levels accompanied by advanced endometrial development. Results thereby enhance understanding regarding the differential effects of MS-IVF on endometrial epithelial and stromal cell development and support the recommendation that frozen embryo transfers should be preferred in women undergoing minimal stimulation with clomiphene citrate to avoid the antagonistic effects of clomiphene citrate on endometrial receptivity.