A study in the latest online issue of Fertility and Sterility has reported that the length of embryo cryostorage does not adversely affect the post thaw survival rate or pregnancy outcome, suggesting it to be a safe and valuable adjunct to assisted reproduction technology (ART).
Sergio Oehninger, Director of the Division of Reproductive Endocrinology and Infertility, Jones Institute for Reproductive Medicine, Virginia, and coworkers, conducted the retrospective study to investigate the effect of cryostorage duration on embryo survival, implantation competence, and pregnancy outcome. The cryopreserved embryos were isolated from in vitro fertilization (IVF) patients, and recipients of oocyte donation cycles. Researchers analyzed a total of 11,768 cryopreserved embryos which had undergone at least one thaw cycle during 1986 to 2007. Post thaw survival proportion and implantation, miscarriage, clinical pregnancy, and live birth rates were considered as the primary outcomes of the study.
The study findings showed that the extent of cryostorage duration for IVF or oocyte donation cycles did not significantly affect the post thaw survival of embryos (frozen at pronuclear or cleavage stages), implantation competence, or pregnancy outcomes. Logistic regression analysis, however, confirmed that factors, such as stage of oocyte, number of embryos transferred and survival proportion, positively predict pregnancy outcomes.
Earlier, Machtinger, et al. (Gynecological Endocrinology, 2002) conducted a case-control study to evaluate the impact of long-term cryopreservation of embryos on its survival, and implantation rate. The study compared 101 women, implanted with their embryos which were cryopreserved for 2–9 years, and the same number of women (control) implanted with embryos stored for 6 months or less. The implantation rate was found to be 4.5% and 5.5% in the study and control groups, respectively. Based on the similar implantation rate observed in both the groups, the researchers concluded that prolonged cryopreservation, do not have an adverse impact on embryo survival and could thereby be an effective option for embryo storage in ARTs.
Embryo cryopreservation plays a major role in the overall outcome achieved by assisted reproduction programs. The procedure also facilitates an improved cumulative success rate for IVF, preventing ovarian hyperstimulation syndrome (OHSS) and reducing the rate of multiple pregnancies. Although several researches suggest that long time preservation of embryos does not affect its survival rate and pregnancy outcome, the maximum duration up to which the embryo can remain in the frozen state for successful implantation, needs further investigation.
1. Riggs R, Mayer J, Dowling-Lacey D, et al. Does storage time influence postthaw survival and pregnancy outcome? An analysis of 11,768 cryopreserved human embryos. Fertil Steril. 2008 Nov 20. [Epub ahead of print]
2. Machtinger R, Dor J, Levron J, et al. The effect of prolonged cryopreservation on embryo survival. Gynecol Endocrinol. 2002 Aug;16(4):293-8.