Use of GnRH antagonists in ovarian stimulation for assisted reproductive technologies compared to the long protocol

The use of GnRH antagonists has revolutionized ovarian stimulation for assisted reproduction. Two GnRH antagonists are clinically available, namely, cetrorelix and ganirelix. Several studies have directly compared these new stimulation protocols against the long GnRH agonist protocol. To evaluate whether there is a reduction in cases of ovarian hyperstimulation syndrome (OHSS) and/or a reduction in pregnancy rates, a meta-analysis was performed. There was a significant reduction of OHSS cases in the cetrorelix studies (odds ratio, OR, 0.23; 95% confidence interval, CI, 0.10-0.54), but no reduction for ganirelix (OR 1.13; 95% CI 0.24-5.31). The incidence of OHSS degree III cases was reduced in the cetrorelix protocols as compared to the long protocol to a nearly significant degree (OR 0.26; 95% CI 0.07-1.01). Ganirelix did not reduce the incidence of OHSS degree III at all (OR 1.08; 95% CI 0.27-4.38). The pregnancy rate per cycle was significantly lower in the ganirelix protocols than in the long protocol (OR 0.76; 95% CI 0.59-0.98). The studies using cetrorelix showed quite similar, not significantly different results for the antagonist and the long protocol groups for the pregnancy rate per cycle (OR 0.91; 95% Cl 0.68-1.22). From the data one can conclude that cetrorelix but not ganirelix will reduce the incidence of cases of OHSS and that cetrorelix but not ganirelix will result in the same pregnancy rates as the long protocol. Several possibilities to explain this phenomenon are discussed.