Endometrial thickness is only part of the picture in IVF. A large study found that the appearance of the uterine lining on ultrasound was also associated with live birth, with a triple-line pattern linked to relatively higher rates.
The analysis covered 45,729 embryo transfer cycles. In frozen embryo transfer, the rise in live birth rates became more gradual at around 9–10 mm, suggesting that thickness and pattern should be assessed together.
The findings suggest that the lining's appearance may be particularly informative when it is relatively thin.
Researchers at the Center for Reproductive Medicine, Tongji Hospital, Huazhong University of Science and Technology in China analyzed fresh and frozen embryo transfers and reported relatively higher live birth rates when the endometrium showed a clearly defined triple-line pattern.
The study was published online in Fertility and Sterility on October 14, 2026.
Triple-line endometrial pattern and the chance of live birth
The team retrospectively analyzed 16,566 fresh embryo transfer cycles and 29,163 frozen embryo transfer cycles performed between January 2018 and August 2022.
Ultrasound appearances were classified as type A or types B/C. Type A showed a distinct three-layer, or triple-line, pattern; that structure was less clearly defined in types B and C.
Types B/C were associated with a lower probability of live birth than type A. After adjustment for multiple variables, relative risks were 0.880 for fresh transfers and 0.892 for frozen transfers. In relative terms, these correspond to approximately 12.0% and 10.8% lower live birth probabilities, respectively.
The researchers suggested evaluating endometrial appearance alongside thickness rather than relying on thickness alone.
Live birth rates of 16.7% and 44.7% in groups differing in both thickness and pattern
Joint analysis of thickness and pattern also identified differences in live birth rates.
Among frozen embryo transfers, the live birth rate was 16.7% in the group with a lining below 7 mm and a type B/C pattern. It was 44.7% in the group with a lining of at least 14 mm and a type A pattern.
The difference was 28.0 percentage points. Because both thickness and pattern differed between these groups, the gap cannot be attributed to either factor alone.
Another important finding was that live birth rates differed by pattern even at similar thicknesses.
In fresh transfers, types B/C were associated with lower live birth rates in some thickness ranges between 7.0 and 8.9 mm. In frozen transfers, the association between a triple-line pattern and higher live birth rates was more pronounced below 10 mm.
These findings suggest that a thicker lining alone does not necessarily indicate the most favorable environment for implantation and pregnancy maintenance.
A thicker lining does not mean a constant rise in live birth rates
Live birth rates did not increase at a constant rate as the endometrium became thicker.
At the thinner end of the range, rates rose relatively rapidly with increasing thickness. After a certain level, the increase became progressively more gradual.
This flattening began at approximately 11–12 mm for fresh transfers and 9–10 mm for frozen transfers.
These values do not establish an optimal thickness, nor do they mean that live birth rates cannot rise beyond those measurements.
Ovarian stimulation method and the duration of hormone administration were associated with endometrial characteristics in fresh transfers. In frozen transfers, the method used to prepare the endometrium was also associated with its characteristics.
Assessment therefore needs to consider the hormonal environment and transfer approach alongside thickness and pattern.
A thin lining does not rule out live birth
The study also shows that a thin lining or the absence of a triple-line pattern does not make pregnancy and birth impossible.
Live births occurred even in the group with a lining below 7 mm and a type B/C pattern.
IVF outcomes depend on many factors beyond the endometrium, including age, embryo development and chromosomal status, uterine conditions and the hormonal environment.
This was an observational analysis of past treatment outcomes. It cannot establish that a triple-line pattern itself directly causes a higher live birth rate.
The researchers concluded that assessing thickness and pattern together may help predict embryo transfer outcomes.
The study supports looking beyond a single thickness measurement to include the structural features visible on ultrasound.
Source: Researchers at Tongji Hospital, Huazhong University of Science and Technology, China. “Joint associations of endometrial thickness and pattern with live birth after fresh and frozen-thawed embryo transfer: a retrospective cohort study of 45,729 cycles.” Fertility and Sterility, published online October 14, 2026. DOI: 10.1016/j.fertnstert.2026.10.008.
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