Recombinant hCG (Ovidrel, Serono, Bari, Italy) 0.25mg will be given if there are 2 follicles >18 mm in diameter. Gonadotrophin injection will not be given on the day of hCG. Agonist trigger will be used if oestradiol concentration on the day of trigger is greater than 15,000 pmol/L or there are more than 15 follicles >16mm on transvaginal scanning. All embryos or blastocysts will be frozen for transfer later following agonist trigger or when the number of oocytes aspirated is 20 or more. Serum FSH, oestradiol, LH and progesterone concentrations are taken 7 days after the Elonva injection and hCG trigger day. Transvaginal ultrasound-guided oocyte retrieval (TUGOR) will be scheduled on Mondays, Wednesdays and Fridays, 34-36 hours after the hCG injection. Flushing of follicles will not be performed. Follicular fluid not contaminated with blood will be collected from the first and largest follicle on each side for estradiol and progesterone level.