Embryo Freezing and Transfer

Introduction

Most people call it embryo freezing, but more correctly it is “embryo cryopreservation” because the ultimate goal is to store the embryo cryogenically and to preserve its viability for future initiation of pregnancy.

Before good embryo cryopreservation protocols were available, most IVF clinics had to destroy any excess embryos left over after embryo transfer. Early embryo cryopreservation programs usually had limited success and usually resulted in the destruction of most of the embryos for which it was attempted. Today, many commercially prepared solutions are available and more successful protocols have been introduced that have made embryo cryopreservation much more successful. Many IVF children now have younger siblings that were conceived at exactly the same time, but born years apart due to the increased success of embryo cryopreservation!

Basic Principles of Embryo Cryopreservation and Thawing

The failure or success of embryo cryopreservation is dependent upon how successful or unsuccessful the removal of water has been from the individual cells of the embryo. If water is left in the cells, it forms crystals when frozen. These crystals act like knives and disrupt the inside of the cells of the embryo or “cut” through the outer layer or “membrane” of the cells. If this cutting or disruption has occurred, the embryo will not survive. In order to avoid the formation of the water crystals, a “cryoprotectant” is added which replaces most of the water inside the embryo. Under the proper conditions, the cryoprotectant will not form crystals and the embryo can safely withstand the drastic reduction in temperature required for cryogenic storage.

When the embryo is removed from cryogenic storage, for use, the reverse process must occur. The cryoprotectant is removed from the cells of the embryo and replaced back with water.

Stage of Embryo Development and Cryopreservation

Different IVF programs cryopreserve embryos on different days of development and all are reportedly successful. The more common stages chosen for embryo cryopreservation are Day 1, Day 2, Day 3 and blastocyst (Day 5-7). The stage of development and day of cryopreservation is usually dependent upon the routine and preferences of the individual laboratories.

Embryo Suitability for Cryopreservation

The embryos most suitable for and most successful for cryopreservation are those that have reached the proper cleavage stage on the day of cryopreservation, have minimal fragmentation and are not undergoing active cleavage. At VFC, after Day 3 embryo transfer, any excess good quality embryos will be considered for cryopreservation. For cryopreservation to occur on Day 3, the embryo must be at least at the 6-cell stage, have an even number of cells and have less then 25% fragmentation. If the embryo has an uneven number of cells or is actively cleaving, cryopreservation will be performed later on Day 3 or on Day 5. If the embryo is very advanced on Day 3 (starting to form a morula stage embryo), it will also be cultured to Day 5 for cryopreservation. Embryos that are growing slowly or that are of questionable quality on Day 3 will be cultured until Day 5 and have the suitability for cryopreservation reassessed at that time. Any embryos that are of poor quality or are in cleavage arrest on Day 3 will not be cryopreserved nor will they be cultured on to Day 5.

Success of Embryo Cryopreservation and Thawing

Many IVF clinics report very good success rates with cryopreserved and thawed embryos. These success rates, no matter how good, are not as good as the pregnancy rates for embryo transfer cycles where the embryos have not been cryopreserved. To date, the best chances for pregnancy occur with embryos that are transferred in the “fresh” transfer. For the patient with good quality excess embryos after transfer, the success of cryopreserved and thawed embryos is sufficiently good that it is well worth pursuing in order to achieve pregnancy should the fresh transfer not be successful or for a second pregnancy should the fresh transfer be a success.

Generally, not all of the embryos cryopreserved and thawed will survive. It is quite realistic to expect 60-70% embryo survival for Day 3 embryos and slightly less for Day 5 embryos. In addition to surviving cryopreservation and thawing, the embryo must also resume growing in order to have the potential of contributing to a pregnancy. At VFC, we thaw Day 3 embryos the day before (late afternoon) the scheduled embryo transfer and monitor the growth of the embryos up until the scheduled transfer time. Day 5 embryos will be thawed the morning of the embryo transfer, which will be scheduled in the afternoon. If available, more embryos may be thawed than required for embryo transfer because of the expected survival rate post thawing.

Length of Cryostorage

Recent reports have described pregnancies that have occurred from embryos cryopreserved and stored for over ten years. The only current limiting factor is for these older embryos is the quality of the cryopreservation protocol in place at the time of storage.