Ice Ice Baby

How long can you keep an embryo frozen?

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A Louisiana woman displaced by Hurricane Katrina gave birth Tuesday to a son whose frozen embryo was rescued from a flooded New Orleans hospital during the 2005 storm. How long do frozen embryos last?

For decades, if you store them the right way. Embryos can remain on ice pretty much indefinitely; one baby was born after being frozen for 13 years. To get that kind of shelf-life, an embryo must be carefully sealed inside a tank filled with liquid nitrogen and monitored to keep it at least 31 degrees Fahrenheit below zero. 


The tanks are usually free-standing and do not have to be refrigerated because the liquid nitrogen inside keeps the contents frozen at about minus 320 degrees. Once all of the nitrogen has turned to gas and seeped out of the tank, the temperature rises dramatically * within 24 hours. To keep the embryo nice and cold, the liquid nitrogen must be checked and refreshed at least weekly. (Some tanks refuel automatically.)  A sealed, undamaged tank can stay at a suitable temperature for a couple of months before being restocked with fresh nitrogen.


Freezing an embryo (embryo cryopreservation) takes a few hours and involves bathing an embryo (usually three or five days after fertilization) in a cryoprotectant solution like sucrose or propylene glycol. The solution draws moisture out of the embryo to prevent ice crystals from forming inside and destroying it. The embryo and solution are then transferred to a thin vial or straw, which is placed in a computerized freezer and taken from room temperature to around minus 18 degrees.


Next, a lab technician begins to “seed the freezing process,” which involves touching a cold object to the straws or vials so that the first ice crystals form in the surrounding solution, and not inside the cell. (There are newer methods of freezing embryos that bypass the seeding phase.) Then the temperature of the embryo is gradually reduced by about half a degree per minute until it reaches somewhere between minus 31 to minus 130 degrees. It’s then safe to plunge the vial containing the embryo into the liquid nitrogen solution for storage.

Once the mother’s uterus is ready to have the embryo implanted, thawing it takes around an hour. The technician soaks the embryo in liquids that gradually wash away the cryoprotectant solution. Once the embryo is thawed to room temperature, it needs to be placed in an incubator to keep it at 98.6 degrees so it can live until implantation. It can last a day or more in this state.

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Explainer thanks John Moschella of East Coast Fertility, Douglas Powers of Boston IVF, and William Venier of San Diego Fertility Center.

Correction, Jan. 17:The original version of this piece said that the temperature inside the tank would drop once all of the nitrogen had turned to gas and seeped out. In fact, the temperature would rise. (Return to the corrected sentence.)