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

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) non-identified 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.