Significantly enhanced pregnancy rates per cycle through cryopreservation and thaw of pronuclear stage oocytes

To examine the results of a 5-year trial using cryopreservation to limit multiple pregnancy and optimize overall pregnancy per cycle.

Retrospective clinical evaluation of pregnancy rates (PRs) per cycle after freezing pronuclear stage human oocytes.

Tertiary care academic center.

Six hundred seventeen patients treated in 776 IVF-ET cycles from January 1987 to December 1991 (less oocyte donation cycles).

Pregnancy rate per cycle after transfer of pre-embryos developed from thawed pronuclear stage oocytes.

Three thousand seven hundred thirty-one oocytes were frozen. Of these, 2,039 were thawed. One thousand three hundred seventy-seven survived thawing (68%), and 1,370 were transferred after passing through syngamy to at least the first cleavage (68%). Of patients with thawing, 359 of 401 (90%) (449 of 505 cycles [89%]) received intrauterine transfer. One hundred thirty-three separate clinical pregnancies were established from 128 different cycles (128/449; 29%); 5 cycles had two thaws, each of which resulted in pregnancy. This PR is less than the overall fresh PR observed in patients who had excess pronucleate oocytes frozen (279/776; 36%) but is remarkably similar when adjusted for the number of pre-embryos transferred per cycle. The age of the patient at the time of cryopreservation and the number of quality of pre-embryos ultimately available for transfer were important factors in the establishment of pregnancy. The mode of ovarian stimulation and duration of cryostorage did not prove meaningful.

Cryopreserved pronucleate oocytes that survive freezing, thawing, and progress through syngamy demonstrate a similar potential for implantation and pregnancy when compared with fresh conceptuses, the cumulative effect of which is an enhanced total PR per cycle.