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The Ins and Outs of In Vitro Fertilization (IVF)

What Happens to the Remaining Embryos?

Some women who go through an IVF procedure will create more embryos than they need. The extra embryos can be cryopreserved (frozen) and used for transfer later. However, sometimes couples decide not to use these extra embryos. These people have the option to have their embryos discarded, donated to research, or donated to another woman to achieve pregnancy.
If you choose to freeze the embryos for future use, it can provide for a second or third opportunity for pregnancy without having to go through the ovarian stimulation and retrieval again. Embryos that meet certain criteria and have developed well can be frozen at several stages of development. Before undergoing the freezing process, the embryos are placed in a cryoprotectant, or antifreeze solution. This solution is made out of sugar and replaces the water within the embryo's cells with a concentrated solution to protect the cells from forming damaging ice crystals.
There are two methods for freezing the embryos, including:
  • Slow cooling: This traditional method consists of placing the embryos into multiple cryoprotectants, added in a specific order, over a 20-minute period. Then, using a computer, the cryoprotectant liquid is slowly cooled until it is frozen. This process takes about two hours and was designed so that the cryoprotectants would infuse into the embryo's cells and protect it from ice. The frozen embryos are then stored in liquid nitrogen (at -196°C or approximately -400°F) or in liquid nitrogen vapor.
Although this method is still widely used, it often results in the loss or damage of embryos upon thawing, compared to the other method of freezing.
  • Vitrification: This newer technique involves an ultra-rapid freezing method, in which the embryos are placed into special solutions and then placed immediately into liquid nitrogen. It uses the same principles as the traditional "slow cooling" method by replacing the water inside the embryo with cryoprotectant. However, instead of a gradual freeze, the embryo is cooled rapidly, which protects the embryo inside the liquid. This process takes approximately one minute.
This final step involves loading the embryo onto a stick and plunging it into liquid nitrogen. Rather than freezing, it is "supercooled" so that the nitrogen becomes solid. The embryo is then suspended in a glass-like bubble of protective liquid. When the vitrified embryos are thawed, they look nearly identical to when they were fresh.
The method used to freeze embryos dictates how they will be warmed and thawed. Not all embryos will survive the freezing/thawing process. Those that do are reassessed. If they have damage that may reduce the chance of implantation, the embryos are not considered viable for transfer.
If you decide to go through the IVF process, it's a good idea to look at the success rates for the fertility clinics you are considering. The expertise and experience of the lab can make a big difference. Although each facility will have various statistics on success rates, some clinics can have survival rates of up to 95 percent for thawing embryos with the vitrification process, with minimal loss of quality to the embryo.
After thawing, the embryos can be transferred into women whose cycle has been synchronized with that of the stage of the frozen embryo.
Last reviewed by: Arthur Schoenstadt, MD
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