Embryo cryopreservation (freezing) is a process where the cells and tissues are frozen for later use. The process of freezing an embryo involves cryoprotectants that prevent the accumulation of salts as water crystallizes during freezing. High concentrations of salts and ice crystals can mortally injure cells. Embryo cryopreservation involves storing the delicate embryo in liquid nitrogen after a controlled freezing using special equipment. Embryos have been thawed successfully after being frozen for 13 years, and pregnancies have been documented from embryos stored for 9 years.
Cryopreservation of embryos allows the couple to have IVF cycles years later if they wish to add to their family. Fertility declines with age, so preserving embryos assures higher quality. During an IVF cycle, between 10 and 30 eggs are recovered from one ovary. Not all eggs are appropriate for cryopreservation, but more eggs increases the number of conception attempts. With multiple preserved embryos, single embryo transfer is a viable option, so there is no risk of multiple pregnancies.
Benefits of Embryo Freezing
Embryo cryopreservation is an important aspect of assisted reproductive technology (ART). The benefits are:
It extends the possibility for pregnancy when fresh cycles fail or when the couple wants additional children after a successful embryo transfer.
It allows the couple to avoid many ethical dilemmas because the embryos are not disregarded.
It offers an alternative to couples who wish to transfer embryos at a later time to avoid the risk of multiple gestation pregnancy.
It increases chance of pregnancy per retrieval cycle by 10-30%.
Embryo Freezing, Thawing, and Survival
Controlled-rate freezing involves slowly cooling the embryos in cryoprotectant fluid. They are stored inside of special labeled plastic vials that are sealed and then placed in containers of liquid nitrogen (dewars). Once frozen, they are placed into labeled tubes. The embryos are thawed out at room temperature, and this takes only a few minutes. The embryo is returned to a culture medium in a petri dish to be assessed for cryodamage and graded based on number of surviving cells.
Embryo survival after freezing is based on the number of viable cells in the embryo after thawing. An embryo has survived if >50% of the cells are still alive and healthy. Around 70% of embryos thawed survive, and embryos with 100% cell survival are considered of equal quality to embryos that were never frozen. However, only around 30% survive with all of cells present. One of the most influential factors regarding embryo survival is embryo morphology, which is the appearance of cells and percentage of fragmentation. In addition, embryos produced using intracytoplasmic sperm injection (ICSI) survive better than those that result from conventional insemination.
Pregnancy Rates with Frozen Embryos
Pregnancy rates are affected by many factors, and like embryo survival after thawing, only 10% of the predictive value is observable and measurable. With frozen embryos, age is not an important factor. For live births, pregnancy rates range from 5% with a single, poor-quality embryo to 35% with four high-quality embryos. The most significant factors that affect pregnancy rates are:
The number of surviving embryos transferred
The number of 100% quality embryos transferred
The morphology scores of the transferred embryos
Blastocyst Survival and Pregnancy Rates
Embryos cultured for five days rather than three are called blastocysts. These embryos are much larger, and have specific needs so damage does not occur during freezing. Many fertility centers will not freeze blastocysts. However, a new protocol has led to a transfer survival rate of 62% for blastocysts, and the pregnancy rate per transfer is now 35%.