Short follicular phase of stimulation following corifollitropin alfa or daily recombinant FSH treatment does not compromise clinical outcome: a retrospective analysis of the Engage trial.

To evaluate whether a short follicular phase of ovarian stimulation compromises the chance of pregnancy, subjects from a double-blind, randomized trial treated with a single dose of corifollitropin alfa (n=756) or daily recombinant FSH (n=750) were categorized as early responders if three follicles ≥17 mm were reached and human chorionic gonadotrophin (HCG) was administered prior to or on stimulation day 8, and as normal responders if three follicles ≥17 mm were reached and HCG was administered after stimulation day 8. In the corifollitropin alfa and recombinant FSH groups, 23.2% and 29.1%, respectively, were early responders (P=0.01). Regardless of the treatment group, the initial ovarian response was higher in early responders, but with two extra days of stimulation, the number and size of follicles on the day of HCG in the normal responders was similar to those of the early responders. The number of oocytes was similar in both response groups following corifollitropin alfa treatment (13.6 versus 14.5) and recombinant FSH treatment (12.8, both groups). The ongoing pregnancy rates were comparable for early and normal responders regardless of the treatment group, supporting successful outcome following a stimulation period of only 1 week.