Whether an ART patient chooses to wait to use the remaining embryos, donates them to others, or simply keeps them frozen without making a decision, a pressing question is: is frozen embryo survival and quality diminished over time? Dr. Ryan Riggs of the Jones Institute for Reproductive Medicine and his colleagues conducted a retrospective study to investigate this question.
The researchers looked at 11,678 cryopreserved embryos during 2,417 cycles, focusing on four main outcomes:
- postthaw survival proportion and implantation,
- clinical pregnancy,
- miscarriage, and
- live birth rates.
Dr. Ryan's group found that there were no significant differences in postthaw survival over time. In addition, the duration of storage had no impact on the rates of implantation, clinical pregnancy, miscarriage, or live birth rates. Both IVF and oocyte donation were examined.
The only factors that the researchers found significant were: oocyte age, survival proportion, and number of transferred embryos. "To our knowledge, we present the largest published analysis of the potential impact of length of storage time on embryo thaw survival and clinically relevant ART outcomes. Human cryopreserved embryos seem to be relatively stable with no obvious or deleterious time effects on pregnancy outcomes." Given the relatively short time over which frozen embryos have been used in infertility patients, there is still relatively little data regarding ongoing health of the children. The authors suggest long-term follow-up be done.
What does this mean for embryos being cryopreserved via vitrification? (Vitrification is the rapid freezing of an embryo such that ice crystals cannot form. With the regular freezing process, ice crystals can cause damage to cells and storage media.) Vitrification is still relatively new in ART, so there is no data to review over time. However, individual clinics that have used vitrification since 2007 or 2008 have already seen a significant increase in pregnancy rates between vitrified and frozen embryos.
Article: Does storage time influence postthaw survival and pregnancy outcome? An analysis of 11.768 cryopreserved human embryos. Fertility and Sterility, Vol 93, No. 1. January 2010.
Authors: Ryan Riggs, MD., Jacob Mayer, Ph.D., Donna Dowling-Lacey, M.S., Ting-Fing Chi, Ph.D., Estella Jones, M.S., and Sergio Oehninger, M.D., Ph.D.