Background: Ebstein’s anomaly and tricuspid valve dysplasia (EA/TVD) are associated with high perinatal mortality. Poor hemodynamic status in both early and late gestation is associated with worse outcome. However, it is not known whether EA/TVD fetuses with more favorable physiology earlier in gestation progress to more severe disease in 3 rd trimester. The goal of this study was to evaluate whether initial echocardiographic indices in fetuses with EA/TVD presenting at gestational age (GA) <24 wks are reliable indicators of physiologic status later in pregnancy. Methods: This multi-center, retrospective study included fetuses diagnosed with EA/TVD from 2005-11 at 23 centers. Patients with initial diagnosis at <24 wks gestation and with ≥2 fetal echocardiograms ≥4 wks apart were included. A core laboratory analyzed the echocardiograms. Markers of poor outcome were defined as absence of antegrade flow across the pulmonary valve, pulmonary valve regurgitation (PR), cardiothoracic ratio >0.48, left ventricular (LV) dysfunction or TV annulus Z score >5.6. Results: The study included 51 fetuses. Median GA at diagnosis was 21 wks [18-24], and the median duration from 2 nd to 3 rd trimester echocardiograms was 12 wks [4-18]. There were 21 fetuses (41%) with >2 markers of poor outcome at <24 wks, which increased to 32 (63%) in later gestation (p=0.001). Nine of 27 fetuses (33%) with antegrade pulmonary blood flow on first echocardiogram developed anatomic or functional pulmonary atresia during follow-up, and 7 of 39 fetuses (18%) without PR initially developed it later. LV dysfunction was present in 2 patients at <24 wks but present in 14 (37%) later (p<0.01). The only variable associated with worsening physiology, from <2 markers of poor outcome in 2 nd trimester to ≥2 markers in 3 rd trimester, was larger TV annulus Z-score (4.1±1.0 vs. 2.4±1.9, p=0.008). Conclusion: These results confirm a wide spectrum of disease presentation and progression in fetuses with EA/TVD. Thus, it is imperative to assess these fetuses serially. In this cohort, there were no early markers of progression other than larger TV annulus Z-score. Great care must be taken in counseling prior to 24 wks, as the absence of factors associated with poor outcome early in pregnancy may be falsely reassuring.