Methods: The present study is a randomized, prospective, controlled trial that was performed on 150 women who referred to two distinct in vitro fertilization (IVF) centers in Iran. Patients were randomly assigned to two experimental groups, as one group was treated with the delayed-start GnRH antagonist protocol (delayed-start group), while another group was treated with the flare-up protocol (flare-up group).

Results: The serum concentrations of estradiol and progesterone, along with the thickness of endometrial tissue and the number of follicles ≥13 mm was significantly increased in the delayed-start group compared with the flare-up group. Also, the number of total oocytes, retrieved mature oocytes, total embryos, fertilized oocytes, as well as the quality of embryos were markedly higher in the delayed-start group when compared with the flare-up group. No statistically significant difference was found in the rates of fertilization, implantation, and pregnancy between the two experimental groups.

Conclusions: According to the above evidence, it seems that the effect of delayed-start protocol on ovarian responsiveness was more pronounced during controlled ovarian stimulation in comparison with the flare-up protocol and the delayed start protocol probably lead to better implantation and pregnancy rates in comparison with the flare up agonist protocol cycle in poor responders.