Impact of different clinical variables on pregnancy outcome following embryo cryopreservation

In our program's 13 years of experience, more than 9000 embryos have been cryopreserved in gonadotropin-stimulated IVF cycles. Over 1500 thaw and transfer cycles have yielded a pregnancy rate of approximately 25%. Different ovarian stimulation regimens (various preparations of FSH, alone or in combination with hMG, with or without concomitant use of a GnRH agonist) did not influence embryo survival or pregnancy rate. Likewise, the application of oocyte/embryo micromanipulation techniques for assisted fertilization (ICSI for male infertility) or assisted hatching (performed selectively) did not have an impact on pregnancy results. Survival and transfer rates of embryos cryopreserved at pronuclear or cleaving stage did not differ significantly. However, implantation and pregnancy rates were higher with pronuclear embryo freezing (day-2 transfers) when compared to embryos frozen at the cleavage stage (day-3 transfers). This may be the result of patient selection and transfer policies. Similar implantation and pregnancy results were achieved in natural and estrogen-progesterone supplemented transfer cycles. Initial experience with pronuclear freezing followed by transfer at the blastocyst stage appears to offer a very successful alternative for selected patients.