How Does Egg Freezing Work?

How Does Egg Freezing Work?

In general, eggs are much more easily damaged during the freezing and thawing process than are embryos.  Due to a variety of biochemical reactions, including hardening of the membrane around the egg and damage to the chromosomes inside that can be caused by freezing, the egg is much more fragile.  For these reasons, the percent of mature eggs that survive freezing and thawing is substantially lower than that of embryos.

A variety of different laboratory protocols have been described in the hope of improving the survival and subsequent successful use of frozen/thawed oocytes.  The most commonly employed protocols are  “slow freezing” and “vitrification”.

The Slow Freeze Process

This process involves gradually dehydrating the oocyte and replacing the water that is normally found inside the egg with a type of “antifreeze”.  This antifreeze is designed to protect the egg as it is slowly frozen.  The eggs are placed in a solution in a programmable freezer while the temperature around them is gradually lowered.  Eventually, the temperature is lowered all the way down to approximately 350 degrees F below zero, where they can be subsequently stored in liquid nitrogen.


The newer methodology of egg freezing, called vitrification, is an ultra-rapid process during which liquids turn into a glass-like solid state, avoiding crystal formation (which could damage the egg’s chromosomes).

Although the amount of data concerning the effect of egg freezing on subsequent offspring is limited, it does appear that cryopreserved oocytes do not have an increased risk of chromosomal abnormalities or other types of birth defects.  Currently, more than 5,000 babies have been born worldwide using frozen oocytes.

Although oocytes cryopreservation offers a substantial amount of promise, there is still a lot of work that must be done in order to bring this technology into the mainstream treatment of infertility.