A 35% cut in risk that is twelve hundredths of one percent
The same true fact, told two ways. One of them sells the test. The other one is what you actually get.
Here is the whole trick of this field in one example, and it is not a trick the companies invented. It is just what happens when you describe a small number two different ways.
The New England Journal of Medicine, 2021: When the risk of a clinical outcome is low, small reductions in absolute risk can correspond to large reductions in relative risk. Consider type 1 diabetes. Our simulations imply a relative reduction in risk of 35% for biologic parents with European ancestries, but the average lifetime risk for type 1 diabetes in the U.S. population is only 0.34%, implying a risk reduction of only 0.12 percentage points.
ASRM reproduces that exact example in its 2026 opinion, which means both the researchers and the American fertility society have put the same sentence on the record.
Here is the published table, for parents of European ancestry, which is the group the scores work best in. Absolute reduction is in percentage points off your baseline risk.
| Condition | US lifetime risk | Absolute reduction | Relative reduction |
|---|---|---|---|
| Type 1 diabetes | 0.34% | 0.12 points | 35% |
| Type 2 diabetes | 35.3% | 5.5 points | 16% |
| Coronary artery disease | 6.7% | 1.1 points | 17% |
| Breast cancer (women) | 12.9% | 1.9 points | 15% |
| Idiopathic short stature | 2.3% | 1.8 points | 77% |
Nothing in that table was measured in a child. The table carries its own footnote saying so: The assessments are based on simulated data and estimates of the within-family predictive power of current polygenic scores… Calculations correspond to a set of embryos for which the lifetime risk of the clinical outcome without ESPS was equal to the lifetime risk or prevalence of the condition in the U.S. population.
These are modelled reductions against US population baseline risk, not results from anybody's pregnancy.
Look at the last row. A 77% relative reduction sounds like the strongest result on the table. The same authors spell out what it is: we calculate that ESPS can reduce the risk of having a child with idiopathic short stature by 1.8% as compared with 10 embryos selected at random. However, with ESPS the expected height of the eventual child would be increased by only 2.5 cm, an outcome that is unlikely to be practically meaningful and that in any case might surprise parents who believe they had successfully selected against short stature.
Two and a half centimetres. That is the 77%.
A second footnote to the same table says: Absolute risk reduction should be a more salient feature of ESPS reports than relative risk aversion.
Now schizophrenia, from a separate modelling paper: Our model shows that a 52% RRR is attainable using the LRP strategy with n=5 embryos. However, this translates to only ≈0.5 percentage points reduction on the absolute scale: a randomly-selected embryo would have a 99% chance of not developing schizophrenia, compared to a 99.5% chance for an embryo selected according to LRP.
Ninety-nine percent, to ninety-nine and a half. Those same authors then add the sentence that should govern every figure on this page: Our risk reduction estimates thus represent an upper bound relative to real-world scenarios.
Three things make the real-world numbers smaller still.
First, the figures assume you have embryos to choose between. The schizophrenia modelling assumes five viable embryos, and its authors state that the benefit drops steeply if the number of embryos falls below n = 5.
The same authors also note that for women of age >42, fewer than 4% of IVF cycles result in live births, making PES impractical.
ASRM makes the same point: Patients should be made aware that IVF often results in a limited number of embryos, thereby limiting their ability to rank embryos.
The NEJM figure for short stature is stated against ten embryos chosen at random, and how many embryos the rest of that published table assumes is not stated in the paper, so this guide attaches no single embryo count to all the figures on this page.
Second, the scenario people imagine, one clearly good embryo and one clearly bad one, is rare. The NEJM authors calculated it: The probability that parents have exactly two viable embryos, one in the top and one in the bottom quintile of polygenic scores, is less than 3%.
Third, the answer depends on the software. A 2024 study in Nature Human Behaviour is titled, in full, Inconsistent embryo selection across polygenic score methods.
Which embryo comes out on top depends on which scoring method the laboratory used.
About the industry's own figure. The company-authored paper most often cited reports a 45-72% reduced risk
for type 1 diabetes. That is a relative figure, and those authors published no absolute figure to go with it. The only absolute figure available for type 1 diabetes is the one at the top of this page: 0.12 percentage points off a lifetime risk of 0.34%.
"What is my child's risk without this test, and what is it with the test? Two numbers."
"How many viable embryos would I need for the figures you just quoted to apply?"