IVF Embryo Attrition: Why the Numbers Keep Dropping After Retrieval

IVF gives you a big number first.

Then it starts taking numbers away.

That is one of the meaner psychological quirks of the whole process.

Quick answer: Attrition is the normal narrowing that happens as follicles become retrieved eggs, mature eggs, fertilized eggs, developing embryos, and finally blastocysts. The amount of attrition varies a lot, so population averages should not be treated as a prediction for your cycle.

Medical note: One IVF cycle can provide useful clues, but it does not always identify a single reason for where attrition happened.

The IVF funnel

Here is the rough sequence:

1. follicles on ultrasound
2. eggs retrieved
3. mature eggs
4. normally fertilized eggs
5. cleavage-stage embryos
6. blastocysts
7. embryos available for freezing, testing, or transfer
8. implantation
9. live birth

The important word is sequence.

An egg has to pass one checkpoint before it can even reach the next.

“But I had 18 follicles. Why did they retrieve 13 eggs?”

Because a follicle is not the same thing as an egg.

Ultrasound lets the clinic see and measure follicles. It does not directly count mature eggs.

Some follicles may not yield an egg at retrieval. Some retrieved eggs will be immature.

“Then why did only 9 fertilize?”

Fertilization is another checkpoint.

Some mature eggs do not fertilize normally even with appropriate sperm exposure or ICSI.

That can be deeply disappointing, but it is part of why IVF starts with the goal of recruiting multiple follicles rather than one.

Then comes the blastocyst wait

This is often the part nobody fully prepares you for.

You may have a reassuring fertilization report, then hear several days later that fewer embryos made it to blastocyst.

Embryo development is biologically demanding.

Some embryos arrest before the blastocyst stage.

Extended culture gives the lab more information about which embryos continue developing, but it also means the number available on day 5 or 6 may be smaller than the number you had on day 3.

⚠️ Be careful with “IVF funnel calculators”

You will find charts online that say:

“Take your egg count, multiply by X%, then Y%, then Z%.”

They are useful for showing the concept of attrition.

They are not personalized predictions.

Age matters. Egg quality matters. Sperm factors matter. Diagnosis matters. Laboratory performance matters. Random biological variation matters.

Even two cycles from the same person can look very different.

Sometimes the pattern matters more than the final number

Your doctor may look at where the drop happened.

For example:

• many follicles but unexpectedly few mature eggs
• good maturity but unexpectedly low fertilization
• good fertilization but poor development to blastocyst

Those patterns can generate questions for the next cycle.

They do not always produce a clean answer.

IVF creates data, not certainty.

💡 What this means

If your numbers drop at each update, that does not mean your cycle is uniquely failing.

Attrition is built into IVF.

What matters is whether you end up with an embryo or embryos that can reasonably be transferred, frozen, or tested.

Should you do another retrieval before transfer?

That is a separate decision and depends on things like:

• age
• ovarian reserve
• number of embryos already banked
• desired family size
• whether genetic testing is being used
• cost
• how physically and emotionally difficult retrieval was

This is where the “right” answer gets very individual.

What happens next in the lab?

Some clinics transfer embryos around day 3.

Many culture embryos longer to day 5 or 6 to see which reach the blastocyst stage.

Next: Day 3 vs Day 5 embryos: why clinics sometimes wait for blastocyst

FAQ

Is IVF attrition normal?
Yes. Not every follicle produces an egg, not every egg is mature, and not every fertilized egg becomes a blastocyst.

Can attrition tell you exactly what went wrong?
Not always. The pattern may suggest questions to investigate, but a single cycle does not necessarily identify one cause.

Can you have a successful cycle with very few embryos?
Yes. Quantity matters, but it is not the only thing that matters.

Sources

• ASRM, Blastocyst Culture and Transfer: https://www.asrm.org/practice-guidance/practice-committee-documents/blastocyst-culture-and-transfer-in-clinically-assisted-reproduction-a-committee-opinion-20182/
• ASRM, Embryology Laboratory Guidance: https://www.asrm.org/practice-guidance/practice-committee-documents/comprehensive-guidance-for-human-embryology-andrology-and-endocrinology-laboratories-management-and-operations-a-committee-opinion-2022/