P-355 Thin endometrium is associated to higher risk of having a displaced window of implantation (WOI)

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Abstract Study question Is thin endometrium (<6 mm) associated to an abnormal receptivity status? Summary answer Thin endometrium (<6 mm) is significantly associated with a displaced WOI. What is known already The measurement of endometrial thickness by 2D vaginal ultrasound (US) is a routine practice in Assisted Reproduction Techniques (ART). Thin endometrium (<6 mm) is associated with poor reproductive outcome; however, the potential reason is unknown. Study design, size, duration Retrospective cohorts study including 25,888 patients in which endometrial lining was measured by 2D US the day before progesterone supplementation in HRT cycles for frozen embryo transfers and Endometrial Receptivity Analysis (ERA) was performed after 5 days of progesterone administration. Endometrial thickness was classified as < 6 mm, 6-12 mm, or > 12 mm; ERA results were considered as receptive or displaced WOI (Pre-receptive, post-receptive, proliferative, and late-receptive) that needs a personalized embryo transfer (pET). Participants/materials, setting, methods Endometrial thickness was measured from one endometrial layer to the other on a longitudinal transvaginal scan at the site of maximum thickness evidenced by 2D US. For ERA analysis, RNA was extracted and sequenced by NGS. Then, the ERA computational predictor obtained the diagnosis as standard or displaced WOI. Main results and the role of chance RA results reveal that women with thin endometrium (<6mm) present significantly higher incidence of displaced WOI (47.49%) than 6-12mm (38.20%) (p = 0.0038), and >12mm (39.75%) (p = 0.026). No other differences were found. *Significant different against other two groups. Global Chi-square test for association P = 0.002. Limitations, reasons for caution This is a retrospective study, having innate limitations of its nature. Wider implications of the findings Thin endometrium (<6mm) is associated to a higher displaced WOI compared to normal lining or hypertrophic endometrium. Normal endometrial thickness (6-12 mm) considered as “receptive” does not preclude a transcriptomic receptivity status in 38.20% of the cases. This finding should be considered when embryo transfer is planned. Trial registration number Not applicable

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