GNRH1 and polycystic ovary syndrome: As for the effect of GnRH-a pretreatment before FET for PCOS patients, nine studies [18, 23, 25, 26, 28, 30, 31, 33, 35] with 1915 participants were included to evaluate endometrial thickness on first day of progesterone supplementation including one RCT [18] and eight case–control studies [23, 25, 26, 28, 30, 31, 33, 35].