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Study question Does a 3-day pretreatment course with a GnRH antagonist in the early follicular phase increase the number of oocytes in a GnRH antagonist stimulation protocol? Summary answer The administration of 3 days of GnRH antagonist before starting ovarian stimulation in a GnRH antagonist protocol increases the number of COCs (Cumulus-Oocyte-Complexes). What is known already The GnRH antagonist protocol is characterized by higher gonadotropin and E2 serum levels at the start of ovarian stimulation (OS), compared with a long pituitary down regulation protocol. The unsuppressed FSH level at the start of a GnRH antagonist cycle allows the initial growth of follicles before addition of exogenous FSH, which may result in asynchrony of the follicular cohort. Menstrual administration of a GnRH antagonist can inhibit follicle growth and improve homogeneity of recruitable follicles. Previous studies showed a trend toward higher numbers of COCs and improved maturation and fertilization rates of retrieved oocytes. Study design, size, duration Retrospective single center crossover study, including consecutive women enrolled in an IVF program in a university hospital from January 2011 to December 2020. All women underwent one standard GnRH antagonist stimulation cycle (“standard cycle”) and one GnRH antagonist stimulation cycle preceded by early administration of GnRH antagonist for 3 days (“pretreatment cycle”). Women with basal progesterone levels >1.5ng/ml, and women undergoing oocyte freezing, oocyte donation or PGT were excluded. In total, 427 patients were included. Participants/materials, setting, methods Women were included when the pretreatment cycle occurred within a time interval of Main results and the role of chance The average female age was 35.1 ± 4.7 years. Indications for fertility treatment included unexplained infertility (34.3%), male-factor infertility (33.3%), age (16.9%), PCOS (8.2%) and endometriosis (2.6%). All cycles were divided into two groups: group 1 (standard, 427 cycles) and group 2 (pretreatment, 427 cycles). The mean duration of stimulation was similar in both groups (10.3 vs 10.3 days, p = 0.2). The starting dose of gonadotropin (196.8 vs 234.9IU, p Limitations, reasons for caution Despite the large dataset, the presence of biases related to the retrospective study design cannot be excluded. Besides, the impact of GnRH pretreatment on pregnancy rate cannot be assessed because of the crossover design. Wider implications of the findings: A 3-day course of GnRH antagonist pretreatment increases the number of COCs obtained after OS. Furthermore, since the initiation of OS in a GnRH antagonist protocol relies on the occurrence of spontaneous menses, addition of three days of GnRH antagonist pretreatment may enhance scheduling flexibility without reducing efficacy. Trial registration number Not applicable