Minimal stimulation
In this study, minimal stimulation protocol is defined as: an extended regimen (from day 3 until the day before triggering) of clomiphene (25-50 mg/d orally) in conjunction with gonadotropin injection starting on cycle days 3 with 75IU daily. No hypothalamic-pituitary suppression was performed, and the final maturation of oocytes was induced by a GnRH-a administration.
When at least one dominating follicle exceeded a mean diameter of 14–18 mm and serum estradiol was 800-1000pmol/L per follicle, 0.1mg of GnRH-a (Triptorelin acetate 0.10 mg, fertipeptil, Ferring, Milan, Italy) were given as trigger, and a transvaginal oocyte retrieval followed 36 hours later.
After in vitro fertilization or intracytoplasmic sperm injection according to the husbands’ semen samples, embryo morphology was assessed on the third day after oocyte retrieval. All available embryos were transferred on day 3 after the retrieval. Remaining supernumerary embryos were vitrified, and no more than 2 embryos were transferred in subsequent naturally prepared frozen cycles.
Luteal phase support of progesterone or micronized progesterone or dydrogesterone were used for 2 weeks until the determination of pregnancy. The serum hCG level was measured by immunofluorescence assay 2 weeks after the embryo transfer.