To discern the potential effect of ovarian stimulation on implantation potential by comparing ongoing pregnancy rates from matched blastocysts in fresh and frozen-thawed single-embryo-transfer cycles.Matched cohort study.Private fertility center.Ninety-three matched pairs of single-blastocyst transfer.Fresh and frozen-thawed embryo transfers were matched on embryo parameters and patient age.Ongoing pregnancy at 10 weeks' gestation.The fresh and frozen-thawed groups did not differ significantly in blastocyst diameter, inner cell mass size, trophectoderm cell count, patient age, use of genetic screening, or presence of supernumerary embryos. The ongoing pregnancy rate was significantly greater in the frozen-thawed group than in the fresh group for transfers of day 6 blastocysts (54.3% vs. 17.1%, respectively), but not for day 5 blastocysts (60.9% vs. 56.5%, respectively). This resulted in the overall ongoing pregnancy rate to be significantly greater in the frozen-thawed group than in the fresh group (55.9% vs. 26.9%, respectively).Autologous day 6 blastocysts transferred in frozen-thawed cycles have significantly greater chance of viable implantation than morphologically equivalent embryos transferred in fresh cycles. This advantage appears to result from impaired implantation of day 6 blastocysts in fresh transfers after ovarian stimulation, suggesting that embryo-endometrium asynchrony is a major cause of impaired endometrial receptivity after ovarian stimulation.