When a woman chooses fertility treatment by in vitro fertilization, the physician creates a number of extra embryos as a safety net. These embryos are usually destroyed, but a technology called embryo storage, or embryo cryopreservation, allows couples to freeze and store the leftover embryos for future attempts at pregnancy. As a result, some familes have siblings born years apart that were conceived from the same batch of embryos.
Although research shows that fresh embryos tend to have a higher success rate, the benefit of embryo storage is that the couple doesn't have to repeat the fertility treatments and processes required to harvest the egg and sperm. In a best-case scenario, up to 70 percent of embryos survive the freezing and thawing procedure. As with egg storage, water molecules may turn into ice when the embryo is placed in the cooling machine. These ice crystals can expand, stretching the embryo, or may act as knives and slice through cell membranes.
To protect the embryo, water is removed and a special freeze-proof cryoprotectant chemical―sort of like a celluar antifreeze―replaces it. When it's time to thaw, this cryoprotectant is replaced with water. Embryos may be stored frozen for an indeterminate amount of time. Some reports have detailed cases in which embryos frozen for up to 10 years have resulted in healthy pregnancies. Among the serious concerns about this technique are who "owns" these extra embryos and what is to become of them when they are unwanted.
The first reported case of a frozen embryo generating a healthy pregnancy was in 1983.
The optimal embryo to freeze is one that is 3 days old.
In some studies, these extra embryos can be adopted by prospective parents.
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