Intrauterine fluid during frozen embryo transfer preparation: Impact of etiology, treatment, and fluid dimension on pregnancy outcomes.

BACKGROUND: Intrauterine fluid (IUF) is detected relatively frequently during frozen embryo transfer (FET) preparation, yet its prognostic meaning remains unclear. We compared outcomes across the causes of IUF and the treatments applied, assessed whether the IUF dimension was associated with outcomes, and compared cycles with and without IUF. METHODS: Eighty-three women with IUF detected during FET preparation and 77 contemporaneous cycles without IUF were analyzed. IUF was measured by transvaginal ultrasonography as maximum anteroposterior diameter and classified by cause (isthmocele, synechiae, idiopathic) and management (no treatment, medical, surgical). Cut-off values were explored by receiver operating characteristic (ROC) analysis, and exploratory logistic regression models for live birth were fitted. RESULTS: Live birth was lower with IUF than in controls (27.7 % vs 42.9 %, p = 0.04). Outcomes did not differ significantly across causes, although the trilaminar pattern (38.5 % vs 69.7 %, p = 0.038) and endometrial thickness (p = 0.02) were lowest in synechiae. Live birth was more frequent in treated than untreated women (38.6 % vs 15.4 %; OR 3.46, 95 % CI 1.20-10.00). The IUF dimension discriminated both outcomes better than chance (AUC 0.668, cut-off 3.10 mm for the pregnancy test; 0.704 and 2.55 mm for live birth), and each 1 mm increase was independently associated with lower odds of live birth (adjusted OR 0.29, 95 % CI 0.14-0.61). CONCLUSION: These exploratory findings suggest that IUF before FET is more than a present-or-absent finding: its dimension was independently associated with live birth. Assessing dimension, etiology, persistence and endometrial pattern together may be more informative than any single parameter.