GNRH1 and ovarian hyperstimulation syndrome: Metformin could reduce the incidence of OHSS (RR 0.46, 95% CI 0.29–0.72), while regarding the clinical pregnancy rate, it demonstrated an increase in women of the GnRH-agonist group (RR 1.32, 95% CI 1.08–1.63), and uncertainty for the GnRH–antagonist group.