Freeze all at the blastocyst or bipronuclear stage A randomized clinical trial

Outcomes using embryos cryopreserved at the blastocyst stage or bipronuclear stage followed by thaw and culture to the blastocyst stage did not differ significantly.

Bruce S. Shapiro, M.D., Ph.D., Said T. Daneshmand, M.D., Forest C. Garner, M.S., Martha Aguirre, Ph.D., Cynthia Hudson, M.S.

Volume 104, Issue 5, Pages 1138-1144


To compare outcomes for patients randomized to have all embryos cryopreserved at the blastocyst stage or at the bipronuclear stage with subsequent post-thaw culture to the blastocyst stage.

Randomized controlled trial.

Private fertility center.

This study included 140 women, age 18–40 years, with at least eight antral follicles, and day 3 FSH <10 IU/L undergoing IVF. Intervention(s):
After oocyte retrieval, subjects were randomized to have entire embryo cohorts cryopreserved at either the bipronuclear stage (2PN Cryo group) or at the blastocyst stage (Blast Cryo group).

Main Outcome Measure(s):
Ongoing pregnancy (viable fetal heart motion at 10 weeks’ gestation) per oocyte retrieval through the first transfer attempt.

No significant differences were observed between the two study groups in age at retrieval, body mass index, antral follicle count, day 3 FSH level, or IVF cycle parameters. No significant differences were observed in ongoing pregnancy rate per retrieval (62.0%; 95% confidence interval [CI], 50.3%–72.4%) in the 2PN Cryo group; and 55.1%; 95% CI, 42.6%–67.1% in the Blast Cryo group), implantation rate (60.0% vs. 62.7%), ongoing pregnancy rate per thaw (62.0% vs. 59.4%), ongoing pregnancy rate per transfer (67.7% vs. 69.1%), and the cumulative ongoing pregnancy rate per retrieval from all thaws to date of embryos derived from the study retrieval cycle (64.8% vs. 60.9%).

Freeze-all at the blastocyst stage or at the bipronuclear stage has similar efficacy and IVF outcomes. The choice between them may depend primarily on logistical factors.

Clinical Trial Registration Number:

  • Daniel J. Kaser, MD

    Dear Dr. Shapiro and colleagues,
    Congratulations on this well-designed RCT, which shows no difference in ongoing pregnancy rates at frozen embryo transfer following elective freeze all cycles according to developmental stage at cryopreservation (2PN vs. blast). While the primary endpoint is certainly important, I found the following observation to be particularly helpful for patient counseling, particularly for cancer patients: the blast cryo group had a significantly greater proportion of patients without embryos available for freezing and an overall greater cancellation rate due to no blastocyst for transfer or poor cryosurvival. In these patients, in whom only a single cycle is often all that is possible, the data suggest a pronuclear freeze may optimize the number of patients who ultimately are able to go to transfer, despite a lower blastulation rate in the thawed group.
    Following this publication, I wonder if you could share your center’s current freeze strategy for a) oncofertility patients and b) those patients who are undergoing elective freeze alls.
    Thanks for your comments and the important contribution!

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