Vitrification

Vitrification is flash-freezing, cooling an egg or embryo so fast that the water inside turns glassy instead of forming ice crystals. It replaced the older slow freezing about fifteen years ago and it is the reason frozen cycles now work about as well as fresh ones.

Where you'll meet it

Anywhere something is being stored. It is how eggs are handled in egg freezing, how embryos are handled in embryo freezing and in a freeze-all cycle, and it is increasingly how sperm is handled too, alongside the older programmed slow freeze.

Why ice is the problem

A cell is mostly water. Freeze it slowly and that water forms crystals, and crystals have edges. In a single large cell like an egg, that damage is often fatal, which is why eggs were for years far harder to freeze than embryos or sperm.

Vitrification gets around it by doing two things at once. The cell is bathed in a high concentration of cryoprotectant, which behaves rather like antifreeze, and then plunged into liquid nitrogen so quickly that the temperature falls by thousands of degrees a minute. There is no time for crystals to organise themselves. What you end up with is a glass rather than a solid block of ice, which is where the word comes from.

What it changed

Survival through the warming step is the headline. With modern vitrification, published figures for thawed eggs are generally quoted somewhere around 85 to 90 per cent, and blastocysts survive at similar or better rates. Slow freezing lost a great deal more than that.

That single change rewrote the rest of the field. It made social egg freezing a reasonable thing to offer rather than a long shot. It made freeze-all protocols practical, which in turn cut the rate of severe overstimulation, because the transfer can wait until the ovaries have settled. And it is why one egg collection can mean several separate chances rather than one.

It also means any success figures you read from before roughly 2012 understate what a modern laboratory does. If you find an old statistic about frozen eggs, check the date before you let it worry you.

What it does not change

Storage time appears not to matter. An embryo frozen five years ago behaves much like one frozen last month, as far as anyone can tell, and nothing ages in liquid nitrogen. An egg collected at thirty-two is still a thirty-two-year-old egg when it is thawed at forty, which is the whole argument for freezing in the first place.

What vitrification cannot do is improve what went into the tank. A poor-quality embryo survives the thaw and is still a poor-quality embryo. Freezing preserves, it does not repair.

How long you are allowed to keep things stored is a legal question rather than a technical one, and the limits differ sharply between countries. Ask your clinic what applies where you are being treated, and ask what happens to your samples if you move, stop paying or change your mind.

Questions to ask

Ask the laboratory for its own thaw survival rate rather than the national figure, and ask how many thaw cycles it has actually done. Experience of warming is the part that matters when it is your sample on the bench, so a solid thaw track record is what to look for. Ask what the storage fee is, how often it is charged, and what the legal limit is where you are being treated.

← Back to the Glossary