Prospective Randomized Comparison of Human Oocyte Cryopreservation with Slow-rate Freezing or Vitrification

Objective: To compare cryopreservation of mature human oocytes with slow-rate freezing and vitrification and determine which is most efficient at establishing a pregnancy.

Design: Prospective randomized.

Setting: Academically affiliated, private fertility center.

Patient(s): Consenting patients with concerns about embryo cryopreservation and more than nine mature oocytes at retrieval were randomized to slow-rate freezing or vitrification of supernumerary (more than nine) oocytes.

Intervention(s): Oocytes were frozen or vitrified, and upon request oocytes were thawed or warmed, respectively.

Main Outcome Measure(s): Oocyte survival, fertilization, embryo development, and clinical pregnancy.

Result(s): Patient use has resulted in 30 thaws and 48 warmings. Women's age at time of cryopreservation was similar. Oocyte survival was significantly higher following vitrification/warming (81%) compared with freezing/thawing (67%). Fertilization was more successful in oocytes vitrified/warmed compared with frozen/thawed. Fertilized oocytes from vitrification/warming had significantly better cleavage rates (84%) compared with freezing/thawing (71%) and resulted in embryos with significantly better morphology. Although similar numbers of embryos were transferred, embryos resulting from vitrified oocytes had significantly enhanced clinical (38%) pregnancy rates compared with embryos resulting from frozen oocyte (13%). Miscarriage and/or spontaneous abortion rates were similar.

Conclusion(s): Our results suggest that vitrification/warming is currently the most efficient means of oocyte cryopreservation in relation to subsequent success in establishing pregnancy.