Age does not affect maximal endometrial thickness achieved in frozen embryo transfer cycles: a SARTCORS study

Chemerinski et al. Reproductive Biology and Endocrinology https://doi.org/10.1186/s12958-025-01451-7 (2025) 23:115 Reproductive Biology and Endocrinology Open Access RESEARCH Age does not affect maximal endometrial thickness achieved in frozen embryo transfer cycles: a SARTCORS study Anat Chemerinski1*, Kristin Blackledge1, Patricia Greenberg2, Nataki C. Douglas1, Peter G. McGovern1 and Sara S. Morelli1 Abstract Background Age is known to affect the success of assisted reproductive technology (ART) treatment. While significant research efforts have been directed at investigating the effects of aging on oocytes, few studies have examined the effect of aging on the endometrium. We sought to assess whether age negatively impacts peak endometrial thickness achieved in frozen embryo transfer (FET) cycles. Methods This was a retrospective cohort study utilizing the Society for Assisted Reproductive Technology Clinic Outcomes Reporting System (SART CORS) database between 2016 and 2020. Young (< 35) and older (≥35yo) nonidentified oocyte donor (NOD) recipients were included to assess the impact of age on endometrial thickness; young and older gestational carriers (GCs) served as the respective controls for these two groups. The primary outcome was peak endometrial thickness achieved in an FET cycle; additional outcomes included cycle cancellation rate, clinical pregnancy rate and live birth rate. Results We observed a weak association between age and endometrial thickness in both NOD recipient and GC cycles. Though pregnancy rates were slightly lower at endometrial thicknesses < 8 mm, we observed no difference in clinical pregnancy rate with endometrial thicknesses between 8 and 18 mm. We found a significantly higher clinical pregnancy rate in GCs compared to NOD recipients in both the young and older age groups, and noted a decreasing clinical pregnancy rate with age in all groups. Conclusion Our data suggest an age-related decline in pregnancy rates in donor oocyte recipients and gestational carrier cycles, in which an endometrial factor would not necessarily be anticipated; this endometrial factor does not appear to be related to endometrial thickness. Therefore, our data support the existence of an endometrial factor that cannot be assessed by measurements of thickness, but nevertheless plays a crucial role in the success of an embryo implantation. Clinical trial number Not applicable. Keywords Endometrial thickness, Age, Frozen embryo transfer *Correspondence: Anat Chemerinski 1 Rutgers Biomedical and Health Sciences, 185 S Orange Ave, Newark, NJ 07103, USA 2 School of Public Health Rutgers Biomedical and Health Sciences, 683 Hoes Ln W, Piscataway, NJ 08854, USA © The Author(s) 2025. 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To view a copy of this licence, visit http://creati vecommons.org/licenses/by-nc-nd/4.0/. Chemerinski et al. Reproductive Biology and Endocrinology (2025) 23:115 Introduction Age is known to affect the success of assisted reproductive technology (ART) treatment as women with advanced maternal age (AMA) have been found to have lower pregnancy and live birth rates and higher miscarriage rates [1]. Significant research efforts have been directed at investigating the effects of aging on oocytes and reproductive outcomes as reproductive aging has largely been attributed to age-related decline in oocyte quality [2, 3]. Despite improvements in understanding and assessing embryo quality, cycles using donor oocytes still only have a live birth rate of approximately 50% per transfer [4], suggesting that other factors are contributing to cycle outcomes. To date, the few studies that have examined the effect of aging on the endometrium have produced inconsistent results [5–7]. A recent analysis of endometrial gene expression demonstrated altered transcriptomic profiles in women 35 years and older [8]. Though this study was limited by sample size and inability to control for other patient characteristics, it suggests an important role for age-related changes within the endometrium in adversely affecting reproductive success. Therefore, the impact of endometrial aging on infertility warrants further investigation. In recent years, frozen embryo transfers (FET) have largely replaced fresh embryo transfers due to concern for adverse effects of ovarian stimulation on the endometrium and increase in utilization of preimplantation genetic testing (PGT) technology [9, 10]. While our current method of assessing endometrial adequacy by measuring thickness in one dimension is crude and limited, it remains our only validated tool. In clinical practice, endometrial thickness is assessed periodically during an FET cycle with the goal of achieving an endometrial thickness of > 7–10 mm in order to optimize pregnancy outcomes [11, 12]. Decreased endometrial thickness (< 8 mm) has been associated with reduced implantation and clinical pregnancy rates [13–15]. Furthermore, a thin endometrium is correlated with adverse neonatal and pregnancy outcomes such as lower birthweight, preterm delivery, placenta previa, and cesarean delivery [16–18]. It is largely unknown whether standard endometrial thickness cutoffs can be applied to older women, and whether older women are as likely as younger women to achieve an optimal endometrial thickness when undergoing ART. One study has evaluated the influence of female age on endometrial thickness, reporting a linear increase in ongoing pregnancy rate for every additional 1 mm of endometrial thickness achieved in women ≥35. However, this was a single-center retrospective study that included all women undergoing FET; as they did not limit their analysis to non-identified oocyte donor (NOD) recipients they were unable to isolate endometrial factor in their analysis [19]. Page 2 of 9 In this study, we used the SART Clinic Outcome Reporting System (SART CORS) cycle data for 2016– 2020 to evaluate the impact of age on maximum endometrial thickness achieved in an FET cycle and to correlate that with clinical outcomes. To isolate the effects of age on the endometr (...truncated)