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

Objective: To examine the results of a 5-year trial using cryopreservation to limit multiple pregnancy and optimize overall pregnancy per cycle. Design: Retrospective clinical evaluation of pregnancy rates (PRs) per cycle after freezing pronuclear stage human oocytes. Setting: Tertiary care academic center. Patients: Six hundred seventeen patients treated in 776 IVF-ET cycles from January 1987 to December 1991 (less oocyte donation cycles). Main Outcome Measure: Pregnancy rate per cycle after transfer of pre-embryos developed from thawed pronuclear stage oocytes. Results: 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 and 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. Conclusions: 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.