Received November 16, 2013
Accepted November 16, 2013

OBJECTIVE: To compare embryo implantation rates between transfer of vitrified embryos in a unstimulated cycle and transfer of fresh embryos in stimulated cycles. METHOD: Retrospective, case-control study.Women younger than 40 years with good ovarian reserve, who have a transfer of at least one grade A embryo. In the study group (52 patients), embryo transfer was differed from a stimulated cycle to an unstimulated cycle. In the control group, 72 patients underwent embryo transfer in a stimulated cycle. We reviewed the medical records of the study group and the control group. We analyzed the variables of interest and significance tests were applied to evaluate the statistical difference between them. RESULTS: The clinical pregnancy rate per transfer was 57.1% in the study group and 61% in the control group. The implantation rate and ongoing pregnancy rate (over 12 weeks) were 34.6% and 42% respectively for the study group, and 36.8% and 41.6% for the control group. There were not significant differences in the variables of interest between the groups. CONCLUSION: In spite of the evidence of lower endometrial receptivity in stimulated cycles, in the present study we found no difference in implantation rates, when embryo-transfers in stimulated cycles were compared with those in unstimulated cycles, of course, choosing appropriately the cases.