Are GnRH antagonists comparable to agonists for use in IVF?

We believe that appropriate comparison of optimal GnRH agonist and antagonist regimens has not been performed yet. Currently available meta-analyses included all comparative studies between GnRH agonists and antagonists performed so far, including less than optimal GnRH antagonist regimens. After critical appraisal of the various studied GnRH antagonist regimens in terms of follicular development and IVF outcome, we postulate that early suppression of endogenous FSH results in optimal follicular development. Additionally, stable and early suppression of LH and progesterone levels during the entire period of stimulation may be an advantage for implantation and pregnancy outcome. In this respect, single dose and particularly flexible protocols seem to be less advantageous. Early FSH and LH suppression can be achieved by early GnRH antagonist administration (stimulation day 1) or by oral contraceptive (OC) pretreatment. More studies comparing long GnRH agonist protocols with 'long' GnRH antagonist protocols, with enough power to identify differences in pregnancy rates, are required before appropriate comparison can be made.