Patients are often able to collect a large number of eggs which result in many viable embryos. Any remaining embryos that are not transferred into the woman’s uterus after a fresh IVF transfer may be frozen or “cryopreserved” in small tubes and kept and stored in the laboratory for future use. Cryopreservation allows the patient to limit the number of embryos transferred “fresh” without discarding the unused embryos that could lead to a future pregnancy. The embryos can be kept in storage for many years. Patients are often able to achieve several pregnancies from just one egg retrieval procedure. These frozen embryos can be transferred back with minimal preparation of the uterine lining. The correct transfer time is calculated based on the stage the embryo was frozen. Aaradhya has tremendous success with this procedure.
Embryo freezing, or cryopreservation, adds an important dimension assisted reproduction by:
- Extending the possibility for pregnancy when fresh cycles fail or when couples want additional children after a successful embryo transfer.
- Avoiding many ethical dilemmas by eliminating the need to dispose of embryos.
- Offering an alternative to couples that might transfer too many embryos and risk a multiple gestation pregnancy.
- Avoiding embryo wastage by freezing embryos individually for efficient use.
- Increasing pregnancies per retrieval cycle with normal outcomes by 10-30% more. Many studies have evaluated the children born from frozen embryos. The result has uniformly been positive with no increase in birth defects or development abnormalities.
We define embryo survival based on the number of viable cells in an embryo after thawing. An embryo has “survived” if >50% of the cells are viable. We consider an embryo to “partially survive” if <50% of its cells are viable, and to be “atretic” if all the cells are dead at thaw. Approximately, 65-70% of embryos survive thaw, 10% partially survive, and 20-25% are atretic. Our data suggests that embryos with 100% cell survival are almost as good as embryos never frozen, but only about 30-35% survives in this fashion.
Embryo morphology (appearance of the cells / percentage of fragmentation) is one of the most influential factors for embryo survival. Interestingly, embryos produced from intracytoplasmic sperm injection (ICSI) also seem to survive somewhat better than embryos produced from conventional insemination.
Embryos that are 2, 4, or 8 cells when frozen have about 5-10% greater survival than embryos with an odd number of cells. Donor egg embryos have a 2-5% greater survival rate than embryos from infertile women when compared by morphology score.
Pregnancy rates are similarly affected by complex relationships and like embryo survival only 7-10% of the predictive value can be observed and measured. Age is not a significant factor with frozen embryos but fewer older women have frozen embryos. From the approximately 20 factors reviewed, the most important factors predicting pregnancy rates are the number of surviving embryos transferred, the number of 100% surviving embryos transferred, and the morphology scores of the transferred embryos.
Blastocysts (embryos cultured for 5 days rather than 2-3) are a special case. The embryos are much larger and have special needs with regard to freezing without damage