Why a PGT Result Rate Is Not an IVF Pregnancy Rate

A PGT result rate and an IVF pregnancy rate often appear side by side in clinic brochures and patient forums, and it is tempting to read them as two steps on one ladder: first the test, then the pregnancy. They are not. One is a laboratory finding about sampled embryos; the other is a clinical event after a transfer. Comparing them directly, or subtracting one from the other, mixes different numerators, different denominators and different moments in time.

This guide walks through what each figure actually counts, why the gap between them can look surprisingly large, and what to ask so that any number you are shown arrives with its definition attached.

Two different questions, two different counts

PGT-A assesses chromosome number in cells sampled from an embryo, and the test has limitations, including the possibility of incorrect findings (HFEA: PGT-A). That sentence already tells you the unit of measurement: embryos, or more precisely biopsy samples from embryos. A PGT result rate answers a laboratory question — of the embryos we sampled, how many produced a usable result, and of those, how many were reported as having the expected chromosome number?

An IVF pregnancy rate answers a clinical question — of some defined group of treatment episodes, how many produced a clinical pregnancy? The unit might be a started cycle, an egg retrieval, or a single embryo transfer. Each choice changes the number.

Neither figure is a statement about the other. A high proportion of embryos reported as chromosomally normal does not tell you how many transfers will happen, and a pregnancy rate per transfer does not tell you how many embryos were available to test.

Step one: unpack the PGT result rate

“PGT result rate” is itself a bundle of at least three separate figures, and clinics do not always say which one they mean.

• Biopsy and laboratory success. Not every sampled embryo yields a reportable result. Some samples fail to amplify or are otherwise uninformative. The proportion with a usable result is one rate.
• Reported chromosome findings. Among embryos with a result, some are reported as having the expected chromosome number, some as having an unexpected number, and some as mosaic — an intermediate finding inferred from the biopsy sample. A mosaic result does not establish the cell composition of the whole embryo, and technical factors can contribute to such findings (ASRM: mosaic PGT-A results).
• Inconclusive or no-result embryos. These are usually reported separately, but sometimes they are quietly folded into a denominator or left out of one. Where they sit changes the percentage.

So when someone says “our PGT normal rate is X%,” the honest follow-up is: normal per what? Per embryo retrieved? Per embryo biopsied? Per embryo with a result? The same laboratory output can produce three noticeably different percentages depending on that choice.

Step two: unpack the IVF pregnancy rate

Clinical outcome rates carry the same ambiguity, and the fertility field has long struggled with it. A clinical pregnancy rate per transfer counts only cycles that reached transfer, which excludes everyone whose cycle stopped earlier. A live birth rate per started cycle counts everyone who began treatment, including those who never reached retrieval or transfer. These are not rival statistics; they answer different questions, and the denominator is the whole story.

For a fuller treatment of how pregnancy, live birth and denominators differ, see IVF success-rate definitions: pregnancy, live birth and denominators. The key point here is narrower: a per-transfer pregnancy rate has already passed through several filters — stimulation response, retrieval, fertilisation, embryo development, testing, and the decision to transfer — before its denominator even begins.

Step three: why the two cannot be subtracted

Put the two figures side by side and the mismatch becomes structural rather than mysterious.

Feature	PGT result rate	IVF pregnancy rate
Numerator	Embryos with a particular laboratory report	Cycles or transfers with a clinical pregnancy
Denominator	Embryos sampled, biopsied or with a result	Cycles started, retrievals, or transfers performed
Time point	Before any transfer	Weeks after a transfer
What it describes	A laboratory finding in a sample	A clinical event in a patient

Because the units differ, “50% normal embryos” and “60% pregnancy per transfer” are not two halves of a whole. You cannot subtract one from the other to estimate your own chances, and you cannot multiply them to predict an outcome. Each describes a different population at a different stage.

There is also a selection effect worth naming. If only embryos reported as having the expected chromosome number are transferred, the per-transfer pregnancy rate is calculated in a group that has already been filtered. That filtering may raise or lower the per-transfer figure relative to an untested group, but the comparison depends on the population, the clinic and the definitions used — and a PGT result does not guarantee a successful pregnancy or a healthy child (HFEA: PGT-A).

A worked illustration, with invented numbers

The figures below are illustrative only. They are not data from any clinic, and they are not a prediction for any patient. Their purpose is to show how arithmetic works when denominators are handled honestly.

Imagine a hypothetical cycle that starts with ten eggs retrieved. Six fertilise and develop into blastocysts suitable for biopsy. Of those six, five yield a reportable PGT-A result, and one is inconclusive. Of the five with results, suppose half — two or three embryos — are reported as having the expected chromosome number. That is a 50% figure, but note its denominator: embryos with a result, not eggs retrieved and not cycles started.

Now suppose one of those embryos is transferred, and the clinic reports a 60% clinical pregnancy rate per transfer. That 60% is calculated across many transfers, not this one. For this hypothetical cycle, the outcome is binary: pregnant or not.

If you instead ask “what proportion of started cycles reached a transfer and a pregnancy?”, the denominator becomes cycles started — and the figure is far lower than either 50% or 60%, because most started cycles in this illustration never produced a transferable tested embryo at all. Nothing has gone wrong mathematically. The three percentages simply answer three different questions.

This is why a clinic can truthfully report a strong per-transfer rate alongside a modest per-started-cycle rate, and why a laboratory can truthfully report a 50% normal rate that feels discouraging next to a 60% pregnancy headline. The numbers are not in conflict; they are measuring different things.

What to ask so each number arrives with its definition

You do not need statistical training to get clarity. A short set of questions, asked once and written down, usually does the job.

1. For the PGT figure: is the denominator embryos retrieved, embryos biopsied, or embryos with a reportable result? How are inconclusive and mosaic results counted?
2. For the pregnancy figure: is the denominator cycles started, egg retrievals, or embryo transfers? Is the numerator a clinical pregnancy or a live birth?
3. Over what time period were these figures collected, and do they describe all patients or a selected subgroup?
4. Are the PGT and pregnancy figures drawn from the same group of patients, or from different periods and populations?
5. How does the clinic report embryos that were tested but never transferred?

These are clinical and laboratory questions, and your treating clinician and genetic counsellor are the right people to answer them for your situation. A coordination service can help you organise records, prepare the question list and route it to the hospital, but it does not interpret results or advise on treatment.

Keeping your own record straight

If you are comparing information from more than one clinic, note the exact wording each one uses, the date of the figures, and whether the denominator was stated or inferred. Approximate dates and quoted phrases are more useful later than a remembered impression. If you are preparing for a consultation, PGT and IVF terminology: questions to ask before you speak with Thai clinicians covers vocabulary that helps you describe what you have already been told.

It also helps to separate what a test can and cannot tell you. Embryo appearance vs genetic test results explains why morphology and PGT can point in different directions, and how to read PGT IVF success rates in Thailand sets out reporting conventions that affect how headline figures are built. For the broader picture of what outcome statistics can and cannot support, Understanding IVF success rates is a useful companion.

The practical takeaway is simple. When you see a PGT result rate, ask what it is a rate of. When you see a pregnancy rate, ask the same. Once both denominators are visible, the two figures stop competing and start doing their proper jobs: describing a laboratory step and a clinical outcome, each on its own terms.