Can cumulative pregnancy rates be increased by freezing and thawing single embryos?

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.

Key Words: Cryopreservation, single frozen embryo, implantation, cumulative pregnancy rate, embryo survival