ObjectiveTo compare the euploid blastocyst formation rates obtained after follicular phase (FP) versus luteal phase (LP) stimulation performed in the same menstrual cycle in a preimplantation genetic diagnosis for aneuploidy testing (PGD-A) program in patients with reduced ovarian reserve.DesignProspective paired noninferiority observational study.SettingPrivate infertility program.Patient(s)Forty-three reduced ovarian reserve patients undergoing a PGD-A.Intervention(s)Both FP and LP stimulations using follicle-stimulating hormone and luteinizing hormone in combination with gonadotropin-releasing hormone (GnRH) antagonist starting on day 2 of the cycle and 5 days after the first oocyte retrieval, respectively, where GnRH agonist was used for both FP and LP ovulation triggering; a trophectoderm biopsy quantitative polymerase chain reaction–based PGD-A strategy; and single euploid blastocyst transfers during a subsequent natural cycle.Main Outcome Measure(s)Primary outcome measure: euploid blastocyst rate per injected metaphase 2 (MII) oocyte; secondary outcome measures: number of cumulus-oocyte complexes (COCs), MII oocytes, and blastocysts.Result(s)Patients with an antimüllerian hormone level of