Egg freezing is an advanced form of assisted reproductive technology. It commonly entails freezing of mature unfertilized human egg. If the egg survive thawing, its then fertilized with a single sperm by direct injection (intracytoplasmic sperm injection or ICSI). In this post I discuss the history, biology and success of egg freezing. No opinions, just the facts ma’am.
Biology. The egg is different from any cell in the body. Its the largest in size and has high water contents. Its surrounding membrane does not allow water to pass easily. Its the only cell in the body that has its DNA spread in the form of a flimsy structure called the spindle (as opposed to other cells where DNA is enclosed in a nuclear membrane)
When the egg is frozen two things take place: water inside the egg form crystals and the concentration of salts inside the egg markedly increase. The two changes disrupt the inner structure of the egg and make the egg swell and sometimes burst. In lieu of its special structure the egg has been a very elusive cell to freeze. Cryoprotectants (Kryo-Protek’tant) are synthetic substances added to the freezing cocktail to reduce these effects.
Short History. There was great interest in freezing human reproductive cells and tissue. Human sperm freezing was available since 1950. It took almost 50 years for egg freezing to be a readily available option. The first successful pregnancy from a thawed human egg was reported in 1986. But it was not until the late 90s that egg freezing was reported routinely and only by few centers worldwide. One non-scientific event helped in popularizing egg freezing, the passage in 1994 of an Italian law that prevents IVF centers from inseminating more than three eggs forced physicians to freeze the rest of the eggs unfertilized. Many of the early studies on egg freezing came from Italy, a fact that limit generalizability of the results of these studies (they should be applied with caution outside Italy). A new method for egg freezing was also proposed recently : rapid freezing (vitrification)
Methods. Two techniques are available slow freezing and vitrification. Simply described vitrification is an ultra-rapid freezing method (drops the temperature from room temperature to –170°C). The water inside the cell changes into a solid state without formation of ice crystals. Preliminary reports on vitrification indicate that this method greatly improve the survival of eggs when they are warmed, when compared to slow freezing. The newer vitrification method require the use of high concentration of cryoprotectants (the compounds that protect the cell from ice formation) and the use of special plastic instrument that allows rapid drop of temperature (most succeful instrument so far is called cryotop).
Clin ic al perspective. Women consider egg freezing in one of two situations fertility preservation for medical indication or fertility extension for social reasons. Egg freezing require stimulation of the ovary with fertility medication for about 10 to 14 days. Conventional medication are used except in estrogen sensitive cancer e.g. breast cancer where special protocols for stimulation are used to reduce estrogen exposure. Vaginal egg retrieval is then performed as usual in IVF. Eggs are frozen four to five hours later using slow freezing or vitrification. When a women comes back to use the eggs e.g. after cancer cure, eggs are thawed and surviving eggs are fertilized using husband or donor sperm through ICSI. Embryo development is observed and embryos are transferred in the cleavage stage (day 3) or blastocyst stage (day 5 or 6). The uterus has to be prepared using estrogen and progesterone to be ready for implantation. Because embryo implantation is lower than that of frozen embryos, usually one extra embryo is transferred than what is usually recommended (e.g. two instead of one embryo)
Success of egg freezing. I reviewed all studies published in the literature world wide regarding egg freezing of non-donor oocytes. The most important number for women considering egg freezing is ongoing and live birth per thawed egg. In other words what is the potential for a pregnancy beyond 20 weeks or a delivery for each egg thawed? This will also allow comparison with the potential for frozen embryo. Please compare the first and third column of the table.
With slow freezing only about 2/3 of the eggs survive freezing compared to 90% with cryotop vitrification. 3/4 of eggs fertilize with slow freezing versus 90% for cryotop. Most importantly about 1.8% of eggs thawed result in an ongoing pregnancy if the egg was frozen with the slow freezing method versus about 7.5% with cryotop vitrification. Because of the limited potential of thawed egg, the procedure is offered to younger women with good response to fertility medication.
Embryo freezing success rate in the US is about 30% per cycle. Each thawed embryo has a 15% chance to produce a live birth (calculated from The Center for Disease Control and Prevention data in women 37 year old or younger). In short cryotop vitrification yields 1/2 the potential for an ongoing pregnancy when compared to frozen embryos, in expert hands. The majority of the studies were conducted in Europe (no large studies from the united states).
Safety of egg freezing. Modern egg freezing does not appear to carry dangers to mother or children. This is however, based on several hundreds to about one thousand children borne using this technology and after a short term follow up. There are no long term data on the health babies borne from frozen eggs e.g . at five years after birth.
In summary egg freezing is an advanced technology that opens new avenues for women and promote their reproductive choice and decision making. Women considering this technology should, however, be aware of its true potential as identified from scientific studies. I dont know of any published studies using egg freezing for fertility extension.