Objective: To determine whether the initiation of P supplementation as artificial luteal phase support (day- 1, day 0, or day + 1 of egg donation) in extensive programs of ovum donation influences cycle cancellation, pregnancy outcome, and implantation rate in day 3 embryo transfers. Design: Prospective randomized trial. Setting: Oocyte donation program at the Instituto Valenciano de Infertilidad, Valencia, Spain. Patient(s): Three hundred recipients with normal ovarian function, absence of uterine anomalies, and undergoing their first egg donation were recruited between September 2003 and September 2004. Intervention(s): A computer-based randomization divided the recipients into three groups when hCG was administered to their matched donors. The first group (group A) started P supplementation the day before oocyte retrieval; the second group (group B) started P supplementation on the day of the oocyte retrieval; and the third group (group C) started P supplementation 1 day after the egg retrieval once fertilization was confirmed. Main Outcome Measure(s): Implantation, pregnancy, and ongoing pregnancy rates were the primary outcome measures considered. The secondary outcome measure was the cancellation rate, especially due to fertilization failure. Result(s): Global cancellation rate and cancellation rate due to fertilization failure were significantly higher in group A (12.4% and 8.2% , respectively) than in group C (3.3% and 0% , respectively). Reproductive outcome was similar in all the groups except for a higher biochemical pregnancy rate in group A (12.9% ) than in groups B (6.6% ) and C (2.3% ). Conclusion(s): Initiation of P on day + 1 of embryo development decreases cancellation rates of day 3 embryo transfers in extensive programs of ovum donation without any deleterious effect on pregnancy outcome or implantation rate. Objective: To determine whether the initiation of P supplementation as artificial luteal phase support (day- 1, day 0, or day + 1 of egg donation) in extensive programs of ovum donation influences cycle cancellation, pregnancy outcome, and implantation rate in day 3 Setting: Oocyte donation program at the Instituto Valenciano de Infertilidad, Valencia, Spain. Patient (s): Three hundred recipients with normal ovarian function, absence of uterine anomalies, and undergoing their first egg donation were recruited between September 2003 and September 2004. Intervention (s): A computer-based randomization divided the recipients into three groups when hCG was administered to their matched donors. The first group (group A) started P supplementation the day before oocyte retrieval; the second group (group B) started P supplementation on the day of the oocyte retrieval; and the third group (group C) started P supplementation 1 day after the e gg retrieval once fertilization was confirmed. Main Outcome Measure (s): Implantation, pregnancy, and ongoing pregnancy rates were the primary outcome measures considered. The second outcome measure was the cancellation rate, especially due to fertilization failure. Result (s): Global cancellation rate and cancellation rate due to fertilization failure were significantly higher in group A (12.4% and 8.2%, respectively) than in group C (3.3% and 0%, respectively). Biochemical pregnancy rate in group A (12.9%) than in groups B (6.6%) and C (2.3%). Conclusion (s): Initiation of P on day + 1 of embryo development decrease cancellation rates of day 3 embryo transfers in extensive programs of ovum donation without any deleterious effect on pregnancy outcome or implantation rate.