What is an embryo?

An embryo can be defined as “a human offspring during the period from approximately the second to the eight week after fertilization (Oxford Living Dictionaries). To produce an embryo in a laboratory, eggs must first be harvested and fertilized. This embryo can either be used for pregnancy or frozen.

What is Embryo Freezing?

Embryo freezing (also known as Embryo Cryopreservation or banking) is a procedure that allows healthy embryos to be preserved at sub-zero temperatures for future use. Embryo freezing is a type of fertility preservation which involves in vitro fertilization (IVF), a procedure in which eggs are extracted from a woman’s ovary and joined with sperm in the laboratory to form embryos. The embryos formed are frozen and can later be thawed and placed in a woman’s uterus. They may also be stored to allow a future pregnancy, donate to others, be used for medical research or training purposes.

Why do people freeze embryos?

Usually, after an in vitro fertilization (IVF) or Intracytoplasmic sperm injection (ICSI) treatment, people have healthy embryos leftover which can’t be used. Rather than discarding them, they can choose to freeze them for future use (in case treatment fails, or other purposes such as embryo donation).

Medical conditions or treatments that could affect or damage your fertility, such as chemotherapy, radiotherapy or surgery, may require people to create embryos to freeze.

How are embryos frozen?

Embryo contains water within their cells, which can form ice crystals when freezes. This expansion can burst the cell, causing it to die. To avoid this occurrence, the embryos are kept in a special protective substance called a cryoprotectant, which replaces water in the cells. Then the embryos are left to incubate in increasing levels of cryoprotectant before they are subjected to freezing.

After removal of most of the water, the embryos are cooled to their preservation state (a state of suspended animation) via one of the two techniques of embryo freezing:

Slow Programmable Freezing (SPF): This process entails protecting the embryos in sealed tubes and then slowly lowering the temperature in the tubes. This process does not allow the embryo cells to age or become damage. However, it consumes time and it is expensive.

Fast freezing or Vitrification: This process occurs so fast, such that the cryoprotected embryos are frozen so quickly that water molecules in the embryonic cells cannot form ice crystals. This protects the embryos and helps in increasing their survival rate during thawing (a reversal of the freezing process).

After freezing, the embryos are stored frozen in liquid nitrogen at a temperature of -1960C in vacuum lined tanks that are computer controlled and monitored, until they are needed.

How long can afrozen embryo be stored?

The maximum period of storage for frozen embryos is uncertain, as this procedure became widely used in the late 1980s. However, in theory, a well frozen embryo can remain viable indefinitely, provided it is stored in liquid nitrogen at a temperature of -1960C, in which no biological processes can exist.

Findings have shown that embryos that have been stored for more than 10 years have successfully generated pregnancies (although most patients tend to use their frozen embryos within 3 to 5 years after they are produced).

Also, some clinics and countries choose to regulate the length of time an embryo can be stored.

IVF Cumulative Pregnancy rates

Outcomes of in vitro fertilization (IVF) treatment are traditionally reported as pregnancies per IVF cycle. However, a couple’s primary concern is the chance of a live birth over an entire treatment course.

The pregnancy rates associated with replacing frozen embryos depend on the age of the patient and the quality of the embryos at the time of cryopreservation. Top-quality embryos from young patients may yield pregnancy rates in excess of 50%, whereas poor-quality embryos may not even survive the thawing process. In some clinics, more than 75% of embryos survive the freeze–thaw cycle.

Several studies reported that the cumulative live-birth rate after 6 cycles was between 72% – 86%.