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 cryopreserva tion to limit multiple pregnancy and optimize overall pregnancy per cy cle. Design: Retrospective clinical evaluation of pregnancy rates (PRs ) per cycle after freezing pronuclear stage human oocytes. Setting: Te rtiary 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 cycl e after transfer of pre-embryos developed from thawed pronuclear stage oocytes. Results: Three thousand seven hundred thirty-one oocytes wer e frozen. Of these, 2,039 were thawed. One thousand three hundred seve nty-seven survived thawing (68%), and 1,370 were transferred after pas sing through syngamy to at least the first cleavage (68%). Of patients with thawing, 359 of 401 (90%) (449 of 505 cycles [89%]) received int rauterine transfer. One hundred thirty-three separate clinical pregnan cies were established from 128 different cycles (128/449; 29%); 5 cycl es had two thaws, each of which resulted in pregnancy. This PR is less than the overall fresh PR observed in patients who had excess pronucl eate oocytes frozen (279/776; 36%) but is remarkably similar when adju sted for the number of pre-embryos transferred per cycle. The age of t he patient at the time of cryopreservation and the number and quality of pre-embryos ultimately available for transfer were important factor s in the establishment of pregnancy. The mode of ovarian stimulation a nd duration of cryostorage did not prove meaningful. Conclusions: Cryo preserved pronucleate oocytes that survive freezing, thawing, and prog ress through syngamy demonstrate a similar potential for implantation and pregnancy when compared with fresh conceptuses, the cumulative eff ect of which is an enhanced total PR per cycle.