P–691 How predictive is endometrial thickness for live birth after fresh and frozen-thawed embryo transfer when no cut-off is employed?

Study question Does endometrial thickness (EMT) predict live birth (LB) after fresh and frozen-thawed embryo transfer (ET) and is there a lower EMT cut-off for ET? Summary answer Once intracavitary pathology and inadvertent progesterone exposure is excluded, EMT is not predictive for LB. EMT is not linearly associated with probability of LB. What is known already EMT is commonly used as a marker of endometrial receptivity and in turn, assisted reproductive technology treatment success. ET is often cancelled or postponed if EMT is below an arbitrary cut-off. However, the available evidence on the relationship between EMT and LB rates is conflicting and too dubious to hold such strong stance. An overwhelming majority of the studies on the subject are retrospective, they use different arbitrary cut off values ranging between 6 to 9 mm with heterogeneous stimulation and transfer protocols. Study design, size, duration Records of all women who underwent fresh or frozen-thawed ET in Koc University Hospital Assisted Reproduction Unit between October 2016 - August 2019 were retrospectively screened. All women who underwent fresh or frozen-thawed blastocyst transfer during the study period were included. Every woman contributed to the study with only one transfer cycle for each category, i.e., fresh ET and frozen-thawed ET. Participants/materials, setting, methods After ruling out endometrial pathology, EMT was measured on the day of ovulation trigger for fresh ET cycles, and on the day of progesterone commencement for frozen-thawed ET. ET was carried out, regardless of EMT, if there was no suspicion of inadvertent progesterone exposure, i.e., due to follicular phase progesterone elevation in fresh or premature ovulation in frozen ET cycles. Main results and the role of chance 560 ET cycles, 273 fresh and 287 frozen-thawed, were analyzed. EMT varied from 4mm to 18mm. EMT were similar between women who achieved a LB and who did not after fresh ET [10.5 (9.2 – 12.2) mm and 9 (8 – 11) mm, respectively, p = 0.11]. Ovarian stimulation characteristics and proportion of women who received a single embryo were similar (69% vs 68.3%, respectively, p = 0.91). Women who achieved a LB was significantly younger than those who did not [35 (32–38) and 37 (33–41), respectively, p < 0.01]. Women who had a LB and who did not after frozen-thawed ET had similar EMT of 8.4 (7.4 – 9.7) mm and 9 (8 – 10) mm, respectively (p = 0.38). Women who achieved a LB were significantly younger than those who did not [32 (29–35) vs 34 (30–38) years, p = 0.04]. The proportion of women who received a single ET was similar between women who achieved a LB and who did not after a FET [86/95 (90.5%) vs 181/192 (94.3%), respectively, p = 0.26]. Area under curve values of EMT for predicting LB in fresh, frozen-thawed and all ET were 0.56, 0.47 and 0.52, respectively. EMT and LB rate were not linearly correlated in fresh or frozen-thawed ET cycles. Limitations, reasons for caution Although our study is retrospective, no women was denied ET due to EMT in our center. Only patients undergoing ET were included in the analysis, which may introduce bias due to the selection of couples who were competent enough to produce at least one blastocyst fit for transfer. Wider implications of the findings: Since women with thin endometrium had reasonable chance for LB even in the absence of a cut-off for EMT in this unique dataset, delaying or denying ET for any given EMT value alone does not seem justified. Further studies in which ET is carried out regardless of EMT are needed. Trial registration number Not applicable