Prognostic factors for the success rate of embryo freezing

To find some prognostic factors for the outcome of frozen-thawed cycles, wehave retrospectively analysed all frozen pre-embryos that were thawedduring 1993 and 1994 at two in-vitro fertilization (IVF) units in Sweden.Supernumerary pre-embryos were frozen from 551 oocyte retrievals and theseresulted in 660 frozen-thawed cycles which lead to 623 thawed embryotransfers. The outcome of these transfers was 137 clinical pregnancies witha pregnancy rate of 22% per frozen-thawed embryo transfers. Women <40years of age had a higher birth rate than those > or =40 years, 19 and5% respectively (P < 0.01). Transfers with two and three pre-embryosresulted in pregnancy rates of 23 and 27%, respectively, compared with 14%for transfer of one embryo. A pregnancy resulting from the initial embryotransfers had a predictive value for results of the subsequentfrozen-thawed cycle. Embryo grade and cleavage stage at the time offreezing was important for the survival of the frozen-thawed pre-embryos.The pregnancy rate was not influenced by the cleavage stage, but a tendencytoward a lower pregnancy rate was seen for the embryos with lower grading.To conclude, cryopreservation seems to be beneficial in women <40 yearsof age, who have supernumerary pre-embryos of good quality for freezing andof which at least two can be transferred.