Uterine Artery PI and RI Before IVF/FET: Why 1.75 Is Not a Pass-Fail Line

PI: mean PI 1.75 is a FET reference point, not a success line

In the 2024 FET study, researchers used ROC analysis to identify a mean uterine artery PI cutoff of 1.75. Patients were then grouped into mean PI >1.75 and mean PI ≤1.75; after 1:1 propensity score matching, 562 patients were included in the analysis.

The practical reading is: in a FET context, mean PI ≤1.75 may be considered a reference point for lower uterine artery resistance.

But it should not be treated as a universal pass-fail number. The same study stated that uterine artery PI on the day of endometrial transformation was not a good predictor of pregnancy outcomes by itself.

So, PI 1.75 is a statistical cutoff from that particular study—not a rule that applies to every IVF or FET cycle. Fresh IVF-ET and FET cycles can differ in protocol and timing, and Doppler values measured on different days should not automatically be compared as if they mean the same thing.

RI: follow the direction, not a universal cutoff

RI is similar in that the direction matters more than a rigid target. A peri-implantation blood-flow study compared pregnant and non-pregnant patients before embryo transfer and found that the pregnant group had lower mean uterine artery RI and PI values. The RI difference had a P value of 0.04, and the PI difference had a P value of 0.003.

That supports the idea that lower uterine artery resistance may be favorable. It does not establish one RI number that can be used as an ideal value for everyone.

If your report lists RI only, the safer interpretation is: a lower RI is a potentially favorable sign; a higher RI should be reviewed together with the rest of the cycle information, rather than used alone to predict transfer success or failure.

A quick way to read the report

Why PI and RI are not enough on their own

Uterine artery PI and RI describe resistance at the uterine artery level. Embryo implantation also depends on the endometrium, the timing of assessment, and blood flow closer to the endometrial and subendometrial region.

In one peri-implantation study, mean endometrial thickness did not differ between pregnant and non-pregnant groups before embryo transfer. But 5–6 days after transfer, the group that conceived had a thicker endometrium, with a statistically significant difference (P = 0.02).

Another IVF-ET study examined endometrial and subendometrial blood flow. When both endometrial and subendometrial flow were visible, the pregnancy rate was 47.8% and the implantation rate was 24.2%. When only subendometrial flow was visible, the pregnancy rate was 29.7% and the implantation rate was 15.8%.

These findings do not mean that any single Doppler number determines the outcome. They show why a transfer assessment should consider uterine artery resistance, endometrial thickness, endometrial/subendometrial blood flow, and scan timing together.

Five things to check before your next appointment

1. Does the report show the left side, right side, or an average? Studies using mean PI may first measure waveforms from both uterine arteries and then calculate an average.
2. Is the mean PI close to or below 1.75? This can be a lower-resistance reference point in a FET context, but it is not a universal success line.
3. Is RI relatively high or low? Research supports lower RI as the more favorable direction before transfer, but there is no single RI cutoff that applies to everyone.
4. On which cycle day was the Doppler performed? PI on the day of endometrial transformation, blood flow before embryo transfer, and findings several days after transfer are not interchangeable.
5. Did the report also assess endometrial thickness and endometrial/subendometrial flow? These findings can fill in gaps that uterine artery PI/RI alone cannot answer.

Bottom line

If you remember one thing, make it this: before IVF/FET transfer, a mean uterine artery PI around ≤1.75 can be used as a lower-resistance reference point in FET, but RI does not have an equally clear, universal “ideal” number.

A higher PI or RI does not mean the transfer will fail. A reassuring PI or RI does not guarantee implantation. The most useful approach is to review the left and right uterine artery values, mean PI, RI, scan date, endometrial thickness, and endometrial/subendometrial blood flow with your fertility specialist in the context of your specific treatment cycle.