Patient resources

Egg freezing: what the numbers can and cannot tell you

Egg freezing is sold with numbers more than almost anything else in fertility, and the numbers are the part that most often mislead. Here is what each one describes, and what none of them can promise.

What freezing actually does

It captures your position at the age you do it. Eggs frozen at 32 remain, biologically, eggs from a 32-year-old, however long they stay frozen.

That is the entire proposition, and it is a real one. It is also narrower than the way it is usually presented, because it preserves the eggs, not the outcome.

The numbers you will be given, and what each means

Your AMH result and antral follicle count. These estimate how many eggs a stimulated cycle is likely to yield. They are planning numbers. They predict the harvest, not the outcome.

The number of eggs collected. A count of what was retrieved on the day.

The number of mature eggs frozen. Lower than the number collected, because not every egg retrieved is mature enough to freeze. This is the number that actually matters and it is often not the one quoted.

Survival rate on thawing. Not every frozen egg survives being warmed. Ask what your laboratory's figure is rather than a general one.

And then the part no number covers. Of the eggs that survive, some will fertilise, some of those will develop into usable embryos, and some of those will be chromosomally normal. That last step is the one nothing in the process can see or improve, and it is governed by the age at freezing.

Each stage subtracts. That is not a flaw in the technique. It is the arithmetic of the biology, and it is why one number in isolation tells you very little.

Why age at freezing matters more than anything else

Two things change with age, at different rates.

Egg number falls. Continuously, from before birth. AMH and follicle counts estimate what remains.

Egg quality falls. Specifically, the chance that an egg carries the correct number of chromosomes. Chromosomal abnormality in the embryo is regarded as the principal cause of implantation failure, and most of those errors arise as the egg matures.

There is no test for quality. Age is the closest available proxy, which is why freezing earlier is more effective, and why it is least appealing precisely at the age it works well.

What the numbers cannot tell you

• They cannot promise a baby. No count of frozen eggs is a guarantee, and any presentation that implies one should be treated with suspicion.
• They cannot substitute for age. A reassuring AMH at 40 means there are eggs to collect. It does not undo the age-related change in quality.
• They cannot predict natural conception. AMH performs poorly at that, and it is routinely over-read as though it could.

When it is worth discussing

Before ovarian surgery that carries a meaningful risk to reserve. ESHRE recommends discussing freezing before ovarian surgery where ovarian endometriosis is extensive. In the same guideline it states that the true benefit of preservation in women with endometriosis remains unknown. Both halves belong in the conversation.

Before treatment that may affect ovarian function. A separate and time-sensitive situation.

When pregnancy is a plan for later rather than now. The common reason, and the one where earlier is genuinely better.

Questions worth asking before you commit

1. How many mature eggs would you expect to freeze per cycle for someone with my results?
2. What is this laboratory's survival rate on thawing?
3. How many cycles would you suggest, and why that number?
4. What happens to the eggs if I later conceive without them?
5. What are the storage arrangements and for how long?

A clinic comfortable with the evidence will answer all five plainly.

The honest summary

Egg freezing is a reasonable option that preserves a real thing. It is not insurance, it does not stop the clock for the eggs you keep using in the meantime, and it works better the earlier it is done.

Anyone presenting it as certainty is describing a product, not the evidence.