P-403 Does a thick endometrium really matter? It’s impact on FET cycles results
Authors: Estefania A , Ainhoa S , Begoña P , Fernando Q , Marcos F , Roberto M 
Published: July 2024

Abstract

Study question
Does a thick endometrium negatively affect the results of FET cycles after PGT?

Summary answer
Endometrial thickness above 10mm did not affect pregnancy rates, and live birth rates were even higher in the group with thick endometrium.

What is known already
The success of an assisted reproduction technique leads on two main points: getting a high-quality embryo and a functional endometrium. The results of the relationship between a thick endometrium and its influence on the pregnancy rates are contradictory till the date, mainly due to the relatively reduced sample sizes and the presence of confoundings. In the same way, they are scarce the papers that analyze the results of FET under hormone replacement therapy in cycles with euploid blastocysts. These cycles could be considered the gold standard to establish the influence of thick endometria in the implantation rate.

Study design, size, duration
Retrospective study of FET cycles under hormone replacement therapy with transfer of an euploid blastocyst after PGT. Multicentric analyze of cycles performed between 2015 and 2021 in our Spanish IVIRMA clinics.

Participants/materials, setting, methods
We analyzed 3158 cycles of FET under hormone replacement therapy with transfer of an euploid blastocyst. Hormone therapy consisted in estrogens (etinil estradiol orally or in patches), plus progesterone (400 mg/ 12 hours, vaginally). Two groups were established: an “optimal” endometrial thickness (7 - 10mm) (80.81%), and those with a higher one (19.19%), the day that the transfer was planned. The objective was to analyze the implantation, miscarriage and live birth rates.

Main results and the role of chance
Both groups were comparable in terms of age, origin an age of the oocytes, origin of the sperm, and serum estradiol levels the day that the embryo transfer was programmed. There were differences in the BMI of the patients (23.00 Vs 23.51, p = 0.004) and the day of the cycle that the embryo transfer was planned (11.8 Vs 11.78, p &lt; 0.001). There were no differences regarding hormonal treatment protocol or dose, nor regarding cycle monitoring.
There were not significant differences in the implantation rate between the group with an endometrial thickness between 7 and 10mm (62.84%; CI 95% 60.90 – 64.78) and the one with a thicker endometrium (66.08%; CI 95% 62.19-69.98; p = 0.143). Neither in the miscarriage rate (15.76%, CI 95% 14.36-17.24; Vs 14.40%, CI 95% 11.70-17.46; p = 0.444).
Statistically significant differences were found in the live birth rate, which was of 47.74% in the group of endometrial thickness between 7 and 10mm (CI 95% 45.75-49.73), and 53.20% in the group with a thicker endometrium (CI 95% 49.09-57.27, p = 0.019)
Taking these results into account, we could conclude that a thick endometrium does not negatively affect the results of FET cycles.

Limitations, reasons for caution
The nature of our study was retrospective not randomized. It was performed with euploid embryos under hormone replacement treatment. Thus, results may not be applicable in other settings. The measure of the endometrial thickness is very variable between professionals and there can be more factors involved in the functionality.

Wider implications of the findings
Embryo transfer should not be canceled in cases of endometrial thickness &gt; 10 mm. More studies are needed to ascertain if it would be beneficial to achieve systematically an endometrial thickness &gt; 10 mm

Trial registration number
Not applicable