The effect of prolonged cryopreservation on embryo survival.

The purpose of this study was to assess the effects of long-term cryopreservation on the survival and implantation rates of embryos. We performed a matched case-control study comparing 101 women whose embryos were transferred after cryopreservation for 2-9 years, with 101 control women whose embryos were transferred after 6 months or less of cryopreservation. A multiple step-wise logistic regression was performed to determine the independent effect of the duration of cryopreservation, patient age and embryo quality on pregnancy and live birth rates. In the study group, 673 embryos were frozen for 24-108 months and of these 451 were thawed. In the control group, 513 embryos were cryopreserved for up to 6 months and 456 were thawed. The implantation rate was similar (4.5% vs. 5.5%) in both groups. We concluded that the duration of cryopreservation did not adversely affect embryo survival, and prolonged cryopreservation appeared to be a safe treatment option.