“…In natural menstrual cycle, the onset of LH surge would be triggered by estradiol (E2) originating from the dominant follicle, exceeding 200–300 pg/mL for a minimum of 50 h. 13 Progesterone rise in circulation would be 12 h before LH surge, which also plays an important role in generating LH surge. 14 , 15 Therefore, several medicines are used as a trigger for inducing an endogenous LH surge (GnRH agonist, 16 progesterone, 17 , 18 and kisspeptin 19 ) or mimicking LH function (human chorionic gonadotropin, hCG 20 , 21 ) and ovulation. After LH surge or hCG, a rapid and robust induction of progesterone receptor (PGR) expression is accompanied by expressions of progesterone biosynthetic enzymes in granulosa cells of preovulatory follicles.…”