High pregnancy rates can be achieved after freezing and thawing human blastocysts.

Objective: To examine the results of a 3-year trial using blastocyst cryopreservation to limit multiple pregnancy and optimize overall pregnancy per cycle.

Design: Retrospective clinical evaluation of pregnancy rates after freezing and thawing human blastocysts.

Setting: Tertiary-care academic center.

Patient(s): Seven hundred fifty-three different patients treated in 783 IVF cycles with blastocysts frozen from July 2000 to June 2003.

Intervention(s): Two thousand, two hundred fifty-nine blastocysts were frozen in cycles in which only blastocysts were cryopreserved (cycles with pronuclear stage oocytes or pre-embryos also cryopreserved were excluded from the analysis). Of these, 628 (27.6%) were thawed in 218 cycles.

Main Outcome Measure(s): Pregnancy rate per cycle with thaw.

Result(s): Four hundred seventy-nine (76.3%) blastocysts survived thawing, and 440 (92.0%) were transferred after exhibiting evidence of survival (most commonly, blastocoele reexpansion). In cycles with a thaw, 211 (96.8%) of 218 underwent intrauterine transfer. An average of 2.09 blastocysts was transferred per replacement. One hundred twenty-five (59.2%) clinical pregnancies were established, which included 23 sets of twins and 5 triplet gestations. Two sets of monozygotic twins were identified after the replacement of a single thawed blastocyst (1.6%). The age of the patient at the time of cryopreservation (<37 years) was an important factor in the establishment of clinical and ongoing pregnancy. The mode of ovarian stimulation, replacement method, and whether blastocysts were frozen on day 5 or day 6 of development did not demonstrate clinical significance.

Conclusion(s): Cryopreserved and thawed blastocysts demonstrated a similar potential for implantation when compared with fresh pre-embryos on day 3. On the basis of these results, the blastocyst stage of development appears to be optimal for clinical freeze-thaw trials.