Study evaluates endometrial thickness and live birth rates in over 244,000 cycles

A 2025 study found that live birth rates increased with endometrial thickness up to about 9 mm, after which they plateaued.

Endometrial thickness is often measured before embryo transfer and is associated with the chance of a live birth.

Generally, a thin endometrium is associated with lower live birth rates, which may have to do with the embryo implanting too close to blood vessels and putting it in an environment that has inappropriately high oxygen concentrations.

This post is a summary of a study by Schmiech et al. (2025), who made use of data from SART between 2019 to 2020, representing over 90% of US clinics. There were a total of 244,001 embryo transfers, with 19.4% being fresh transfers, 39.4% being frozen transfers without PGT and 41.2% frozen with PGT. SART requests the “most recent” thickness from member clinics — so it’s not clear exactly when the lining was measured, and likely varied between clinics. Lining thickness ranged from more than 1 mm to less than 30 mm:

• <6mm: 2,585 cases (1.1%)
• 6-7mm: 7,050 cases (2.9%)
• 7-8mm: 26,935 cases (11.0%)
• 8-9mm: 50,041 cases (20.5%)
• 9-10mm: 46,829 cases (19.2%)
• ≥10mm: 110,561 cases (45.3%)

While you’re reading this, you might benefit from checking out the glossary term for odds ratios (doesn’t count toward the monthly paywall limit). I’ve recently updated it and included a section on how to mathematically transform odds ratios back to live birth probabilities, so hopefully it clears things up a bit!

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Live birth rates plateau with an endometrial thickness of 9 mm

Rather than present the raw data from over 244,000 embryo transfers, the researchers incorporated this data into a statistical model so their results could be more clearly represented in a graph.

The graph is copyrighted, so I’ve done my best to estimate the values for each endometrial thickness value below. Their graph goes up to 30 mm thickness, but I stopped at 15 mm in my chart. The live birth rate plateaus at about 46-47%, starting at an endometrial thickness of 9 mm.

Wow! 1 mm had a 18% chance of live birth? Based on this model, yes, but I don’t know if this study actually included patients with a 1 mm endometrium. In fact, they state that they excluded patients with 1 mm or less (and with 30 mm or more), and that there were 2,585 cases (1.1%) with a lining <6 mm. So maybe, but it’s not clear! Another point to be made is that they didn’t see a decrease in live birth rates with a lining that was too thick.

Since the live birth rates peak at 9 mm, the researchers considered this to be the cutoff.

So next, they evaluated success rates for those with ≤9 mm and >9 mm thickness. They used odds ratios to show how live birth rates changed with a 1 mm difference in endometrial thickness, which were statistically adjusted to account for changes in patient age, BMI, etc.

• Fresh Cycles (n= 47,333): For endometrial thicknesses up to 9 mm, each 1 mm increase was associated with a 15% increase in the odds of live birth (adjusted odds ratio: 1.15, 95% CI: 1.09-1.20), while increases above 9 mm provide a smaller benefit, with each additional mm increasing the odds by 4% (adjusted odds ratio: 1.04, 95% CI: 1.02-1.06).
• Frozen Cycles without PGT (n= 96,249): For ≤9 mm, each mm increase raises the odds of live birth by 13% (adjusted odds ratio: 1.13, 95% CI: 1.09-1.16), but for >9 mm, each mm increase only results in a 2% increase (adjusted odds ratio: 1.02, 95% CI: 1.01-1.04).
• Frozen Cycles with PGT (n= 100,419): For ≤9 mm, each mm increase raises the odds of live birth by 19% (adjusted odds ratio: 1.19, 95% CI: 1.16-1.22), but for >9 mm, each mm increase only results in a 2% increase (adjusted odds ratio: 1.02, 95% CI: 1.00-1.03).

Considering endometrial thickness and embryo stage

Next, the researchers calculated odds ratios (as above), but this time considered the stage of embryo development — ie. did endometrial thickness influence cleavage stage (day 3) or blastocyst stage live birth rates?

Fresh cycles

• Cleavage stage embryo transfer:
  • ≤9mm: 16% increase (adjusted OR: 1.16, 95% CI: 1.08-1.24)
  • >9mm: No statistically significant change
• Blastocyst Transfer:
  • ≤9mm: 13% increase (adjusted OR: 1.13, 95% CI: 1.09-1.18)
  • >9mm: 6% increase (adjusted OR: 1.06, 95% CI: 1.05-1.07)

Frozen Cycles without PGT

• Cleavage stage embryo transfer:
  • ≤9mm: 15% increase (adjusted OR: 1.15, 95% CI: 1.04-1.28)
  • >9mm: No statistically significant change
• Blastocyst Transfer:
  • ≤9mm: 12% increase (adjusted OR: 1.12, 95% CI: 1.09-1.14)
  • >9mm: 3% increase (adjusted OR: 1.03, 95% CI: 1.02-1.04)

Frozen Cycles with PGT

• Blastocyst Transfer:
  • ≤9mm: 18% increase (adjusted OR: 1.18, 95% CI: 1.15-1.20)
  • >9mm: 2% increase (adjusted OR: 1.02, 95% CI: 1.01-1.03)

Conclusions

The researchers found that live birth rates increased linearly from 1 mm to 9 mm. At 9 mm, the chances of live birth started to plateau and didn’t increase much past 47%.

Aside from these main results, they also looked at outcomes for fresh cycles, frozen cycles (with and without PGT), and for cycles with cleavage or blastocyst stage embryo transfers. Live birth rates reached a maximum around a thickness of 9 mm for blastocyst transfers, with only minor increases past this point. There were no further increases for cleavage stage transfers past 9 mm.

Previous research tends to agree with this study, particularly the results of a 2022 study I covered in my post Endometrial thickness and pregnancy outcomes after 96,000 embryo transfers. However, not all studies show that endometrial thickness peaks, or is associated with live birth rates.

The authors note that even though some people may not reach 9 mm endometrial thickness, pregnancies can still occur with a thinner endometrium and these results shouldn’t stop anyone from transferring!

Related studies

To learn more about this topic, you can check out a number of studies referenced in this study below (3 links):

Reference

Schmiech K, Li M, Chen LX, Dow MP, Baker VL. Association of Endometrial Thickness with Live Birth Rate: a Study Utilizing the Society for Assisted Reproductive Technology Clinic Outcome Reporting System. Fertil Steril. 2025 Jan 2:S0015-0282(24)02467-1. doi: 10.1016/j.fertnstert.2024.12.032. Epub ahead of print. PMID: 39755137.

 

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About Embryoman

Embryoman (Sean Lauber) is a former embryologist and the founder of Remembryo, an IVF research and fertility education website. After working in an IVF lab in the US, he returned to Canada and now focuses on making fertility research more accessible. He holds a Master’s in Immunology and launched Remembryo in 2018 to help patients and professionals make sense of IVF research. Sean shares weekly study updates on Facebook, Instagram, and Reddit regularly. He also answers questions on Reddit or in his private Facebook group.

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