Objective: To evaluate the extent to which transfers of frozen single embryos increase cumulative pregnancy rates.
Design: Retrospective analysis.
Setting: IVF unit of a university hospital.
Patient(s): Patients undergoing IVF cycles that were carried out from 2001 to 2005 (n = 1758). Patients were assigned to three groups according to the number of embryos frozen: group A, no cryopreservation; group B, a single embryo frozen; group C, several embryos frozen.
Intervention(s): Analysis of fresh ETs as a function of the number of embryos frozen and comparison outcomes for the thawing of a single embryo between subgroups B* (only one embryo frozen and thawed) and C* (last embryo of the cohort thawed).
Main outcome measure(s): Implantation and pregnancy rates after fresh ETs and embryo survival and pregnancy rates after the transfer of a single thawed embryo.
Result(s): The pregnancy rate per fresh ET increased significantly with the number of embryos frozen: 16.2% in group A, 21.4% in group B, and 26.5% in group C. For single thawed embryos, survival was higher in group C* (91.7%) than in group B* (72.6%). The pregnancy rate was also significantly higher in group C* (19.4% vs. 0%).
Conclusion(s): The freezing of single embryos is of no benefit in cumulative pregnancy rates. ET strategies should therefore be reviewed.