['To compare pregnancy rates following the transfer of thawed frozen embryos according to the type of GnRH antagonist or agonist used during controlled ovarian hyperstimulation (COH).', 'Retrospective review of frozen embryo transfers according to whether a GnRH agonist or antagonist was used. Furthermore to determine if a specific antagonist/agonist resulted in higher pregnancy rates than the other.', 'The pregnancy rates in two different age categories were similar whether the COH regimen used the GnRH agonist leuprolide acetate or the GnRH antagonist cetrorelix. However, lower pregnancy rates were found with the GnRH antagonist ganirelix.'] Is evidence that the main adverse effect of ganirelix on pregnancy and implantation rates on the embryo rather than the endometrium?