Potential factors affecting embryo survival and clinical outcome with cryopreserved pronuclear human embryos

Abstract

Objective: This study was undertaken to determine whether the method of fertilization has a significant impact on survival and/or clinical pregnancy rates of cryopreserved human pronuclear (2PN) stage embryos.

Design: A retrospective analysis of cryosurvival and clinical pregnancy rates after thawing of 2PN stage embryos from January 2000 through December 2002 in a private Assisted Reproductive Technology (ART) center.

Material and methods: A total of 1408 human 2PN embryos were cryopreserved using a Planer Kryo 10 Series III freezing unit (TS Scientific, Perkasie, Pa) after dehydration/equilibration through Propanediol (Sigma Chemical, St. Louis, Mo) and sucrose. On thawing, embryos were cultured in vitro with P-1 medium with 10% Serum Substitute Supplement (Irvine Scientific, Santa Ana, Calif). Embryo transfer was performed at 40 to 48 hours from time of thaw into a recipient uterus after standard estradiol/progesterone preparation.

Results: In 2000, 78% of all frozen 2PN embryos survived and were transferred in 181 cycles producing a delivery rate of 26% per transfer. However, 59% of these cycles were intracytoplasmic sperm injection (ICSI), and the survival of frozen 2PN from these cycles (72%) was lower than the respective survival of frozen 2PN embryos from in vitro fertilization (IVF) (81%; P<.025). Changes to protocols for thawing frozen 2PN embryos were therefore explored and implemented during 2001, resulting in equivalent survival rates of frozen 2PN embryos from IVF and ICSI during 2001 (78% and 80%, respectively) and 2002 (73% and 74%, respectively). Coincidentally, the proportion of all cycles that were performed with ICSI increased (73% in 2001 to 78% in 2002; P<.01) and pregnancy rates after transfer of frozen/thawed 2PN embryos from ICSI increased from 15% in 2000 to 30% in 2002.

Conclusion: 2PN stage embryo cryosurvival may be negatively affected by ICSI, possibly caused by disruption of the zona pellucida and vitelline membrane before cryopreservation, and/or because ICSI promotes fertilization of some compromised eggs (producing compromised 2PN embryos) that would not have fertilized by conventional IVF. Without close attention to embryo freezing and thawing protocols relative to outcome, lower cryosurvival of unselected ICSI-produced embryos can negatively impact pregnancy outcomes.