How freezing actually works, and what it means for your eggs and embryos

Freezing has quietly become the centre of modern fertility treatment. Eggs are frozen for later. Embryos are frozen while genetic tests run. Whole cycles are frozen to allow the body to return to baseline before a transfer. If you are treating abroad, freezing is very often what turns your treatment into two trips.

For something so central, it is explained remarkably little.

Vitrification, in plain terms

The problem with freezing living cells is ice. Ice crystals form as water freezes, and crystals damage cell structures. Slow freezing, the older approach, gave them time to form.

Vitrification is the technique that replaced it. The cells are treated with cryoprotectants and cooled extremely fast — fast enough that the water does not have time to crystallise. It solidifies into a glass-like state instead, which is where the name comes from. Then the material is stored in liquid nitrogen at around minus 196 degrees Celsius.

Two things follow from this that are worth knowing.

Speed and handling are the whole technique. Vitrification depends on precise timing and careful work by an embryologist. This is a genuine skill difference between laboratories, and one of the reasonable things to ask about when you are choosing a clinic.

Nothing degrades in storage. At that temperature, biological activity effectively stops. Material stored for five years is not “older” than material stored for one. What matters is your age when it was frozen, not how long it has sat there.

Eggs and embryos behave differently

Eggs are harder to freeze. An egg is a large cell with a lot of water in it, which is precisely what makes ice a problem. Vitrification improved this dramatically — it is why egg freezing became viable at all — but eggs remain more delicate than embryos.

Embryos tolerate it better. They are made of many smaller cells, and losing some is survivable in a way that damage to a single egg is not.

This is behind a question many people face when freezing for the future: eggs, or fertilise now and freeze embryos? It is not only technical — embryos involve decisions about whose sperm and about what happens to them later, which is its own subject in the embryos you do not use. But the technical difference is real, and your clinic should explain how it applies to you.

Thawing

Warming is as controlled as the cooling was, and it happens shortly before use.

Survival rates are high with modern vitrification but they are not one hundred per cent, and this is worth hearing before it is relevant rather than after. Ask your clinic what proportion of eggs and embryos typically survive warming in their laboratory. A clinic that answers directly is telling you something good about itself.

It is also why the number you freeze is discussed in terms of a realistic plan rather than a single item. Each stage narrows the numbers, and freezing is one of those stages.

The questions to ask about storage

These are the practical ones, and they are easy to skip in the middle of a cycle:

• Where exactly is my material stored, and under whose name?
• What does storage cost, how often is it billed, and how am I invoiced? Storage fees are a recurring cost that outlasts the treatment, and they are one of the items missing from most quotes — see what a treatment quote does not cover.
• How long can it be stored, and does any limit apply here?
• What happens if I stop paying, or you cannot reach me? Ask what the clinic does before taking any action, and make sure your contact details are current.
• What consents have I signed, and can I change them later?
• What would moving my material to another clinic or country involve? Worth knowing in outline before you need it — the shape of it is in moving frozen embryos and eggs across borders.

The one people wish they had asked

Keep your own record of what is stored. How many eggs or embryos, frozen on what date, at what stage, under what identifiers, at which facility. Clinics keep records, but clinics also merge, change systems and occasionally close, and you may be making decisions about this material in ten or fifteen years.

It takes five minutes at the time and is close to impossible to reconstruct later.

This article is general information, not medical advice. What is stored for you, and what your options are, are questions for your clinic and your signed consents.

For how storage fits into the wider process, read the patient journey.