Our working hypothesis is that patients undergoing "in vitro" fertilization (IVF) with higher response to ovarian stimulation protocols recover a higher number of oocytes and, this elevated response could be translated into increased incidence of chromosomally abnormal embryos. Our objective is to develop a prospective study on healthy young donors, with a previous cycle with high ovarian response (>20 oocytes and/or E2 levels the day of the hCG injection >3000 pg/mL), but without developing mild or severe hyperstimulation syndrome. After signing a proper written consent, these donors would agree to undergo two subsequent stimulation cycles following two different protocols. In one cycle the stimulation pattern would be similar to the previous one, with elevated response and, in another cycle the amount of gonadotropins would be cut down in order to obtain lower ovarian response. Oocytes obtained in each cycle would be donated to anonymous recipients and after fertilization, embryo quality and chromosomal status of the resulting embryos would be evaluated. Preimplantation genetic diagnosis (PGD) will be performed on day-3 embryos and chromosomes 13, 15, 16, 17, 18, 21, 22, X and Y would be analyzed by fluorescence "in situ" hybridization (FISH).

Inclusion Criteria:

• Healthy women between 18 - 29
• with previous cycle with standard dosis of Gonadotrophin and high response to the treatment (> 20 oocytes)
• No symptom of OHSS

Exclusion Criteria:

• donors with 2 previous miscarriages
• PCO
• Severe Male Factor