From the Guidelines
The optimal endometrial thickness for frozen embryo transfer is not a determining factor for the decision to perform embryo transfer, as the decision should not be based on endometrial characteristics 1. When considering the preparation for frozen embryo transfer, the focus is on achieving an endometrial environment that supports implantation, rather than a specific thickness.
- Estrogen supplementation is commonly used to prepare the endometrium, with options including oral estradiol, estradiol valerate injections, or estrogen patches.
- Progesterone is added once adequate endometrial preparation is achieved, with common forms including vaginal progesterone suppositories, progesterone in oil injections, or oral micronized progesterone.
- The timing of embryo transfer is synchronized with the start of progesterone, typically occurring 5-6 days after initiation for blastocyst transfers. While endometrial thickness is monitored during preparation, the decision to proceed with embryo transfer should not be based solely on this factor, as indicated by the most recent guidelines 1. Instead, the focus is on creating an optimal environment for implantation, with endometrial thickness being just one aspect of this preparation.
- Studies have shown that other factors, such as estradiol levels and progesterone levels, are also important considerations in IVF outcomes 1. However, the most recent and highest quality study emphasizes that endometrial characteristics should not dictate the decision to perform embryo transfer 1.
From the Research
Optimal Endometrial Thickness for Frozen Embryo Transfer
- The optimal endometrial thickness for frozen embryo transfer (FET) is typically considered to be around 7-8 mm on the day of ovulation before progesterone administration, as reported in a study published in the Journal of Clinical Medicine 2.
- However, another study published in Reproductive Sciences found that baseline endometrial thickness or endometrial thickness change in response to estrogen is not predictive of FET success in medicated cycles, with no significant difference in endometrial thickness between successful and unsuccessful transfers 3.
- A review of the current literature on FET preparation methods, published in Human Reproduction, proposed a standardized timing strategy for FET, but noted that the optimal endometrial preparation protocol is yet to be determined and requires further research 4.
- A retrospective study published in PLOS ONE found a significant correlation between endometrial thickness and clinical pregnancy outcomes, with an optimal endometrial thickness range of 8.7-14.5 mm for live birth rate 5.
- An update on clinical practices for preparing the endometrium for FET, published in Reproductive Biology and Endocrinology, highlighted the increasing interest in individualized endometrial preparation and the need for further research on the ideal method of endometrial preparation 6.
Factors Influencing Endometrial Thickness and FET Outcomes
- Maternal age, embryo transfer protocol, and endometrial thickness on the day of embryo transfer have been identified as factors influencing cycle outcomes 2.
- Body mass index (BMI) has been found to have a statistically significant weak positive linear relationship with baseline endometrial thickness 3.
- The method of endometrial preparation, including hormone replacement therapy and natural cycle FET, may influence FET outcomes, with some studies suggesting a "back to nature" approach using natural cycle FET in ovulatory women 6.
Endometrial Thickness and Pregnancy Outcomes
- A significant correlation has been observed between endometrial thickness and implantation rates, clinical pregnancy rates, and live birth rates, with optimal outcomes achieved when endometrial thickness is within a certain range 5.
- Endometrial thickness less than 8.7 mm has been associated with reduced implantation, clinical pregnancy, and live birth rates, while endometrial thickness greater than 14.5 mm may also reduce live birth rates 5.