Why the numbers go down: follicles, eggs, mature eggs, embryos

Almost everyone going through IVF has the same experience: each phone call from the lab brings a smaller number than the last. That funnel is normal biology, not a sign that something went wrong. Here is what happens at each step and roughly what to expect.

Follicles → eggs collected

On your scans the doctor counts follicles — the fluid-filled sacs on the ovary that may contain an egg. Not every follicle does, and small ones often don't. On average about 70–80% of the follicles measured on the last scan yield an egg. A follicle can also be hard to reach or may already have released its egg.

Eggs collected → mature eggs

An egg must be mature (called "MII") to be fertilised. Immature eggs are seen in almost every collection. Typically 70–85% of eggs are mature. Immature eggs are usually not usable; occasionally they mature in the dish over a few hours and can be used, but their chances are lower.

Mature eggs → fertilised eggs

Fertilisation is checked the next morning. With ICSI, around 70–80% of injected mature eggs fertilise normally; with conventional IVF the range is similar or slightly wider. Some eggs don't fertilise at all; some fertilise abnormally (one or three sets of genetic material) and are not used. Complete failure of fertilisation is uncommon but does happen, and your clinic will discuss why.

Fertilised eggs → embryos on day 3

Nearly all normally fertilised eggs divide over the next two days. 90–95% reach the day-3 stage. Some will already show slower or uneven development.

Day-3 embryos → blastocysts (day 5–6)

This is the biggest drop, and the most misunderstood. Only embryos with the right genetic and cellular make-up can keep going, so around 40–60% of fertilised eggs become blastocysts in a good lab. The ones that stop were, in most cases, never able to become a pregnancy — the lab is watching biology sort itself, not causing the loss.

Blastocysts → good-quality, usable blastocysts

Of the blastocysts, some will be graded good enough to transfer or freeze and some won't. Depending on age this might be most of them or only a few.

What this looks like in practice

A common pattern for someone in their early thirties: 12 follicles → 10 eggs → 8 mature → 6 fertilised → 6 day-3 embryos → 3 blastocysts → 2 usable. For someone in their early forties the same start might end with one, or none, mainly because a higher share of eggs carry chromosomal errors.

Why labs track these percentages

The steps above are exactly the numbers a good laboratory measures for itself every month — fertilisation rate, cleavage rate, blastocyst rate — and compares with benchmarks. When a lab's numbers stay in the expected range across many patients, the drop you see is your biology working as biology does, not the lab.