Ebstein anomaly (EA) and tricuspid valve dysplasia (TVD) are rare congenital malformations associated with nearly 50% mortality when diagnosed in utero. The diseases often produce severe tricuspid regurgitation (TR) in the fetus and in some cases, pulmonary regurgitation (PR) and circular shunting ensue. Since the ductus arteriosus (DA) plays a critical role in the circular shunt and may be constricted by transplacental non-steroidal anti-inflammatory drugs (NSAIDs), we sought to assess the effect of NSAIDs on fetuses with EA/TVD. We reviewed mothers of singleton fetuses with EA/TVD and PR, indicative of circular shunting, who were offered NSAIDs at multiple centers from 2010-2018. Initial dosing consisted of indomethacin, followed by ibuprofen in most cases. Twenty-one patients at 10 centers were offered therapy at a median gestational age (GA) of 30.0 weeks (range: 20.9-34.9). Most (15/21=71%) mothers received NSAIDs, and 12/15 (80%) achieved DA constriction after a median of 2.0 days (1.0-6.0). All fetuses with DA constriction had improved PR; 92% had improved Doppler patterns. Median GA at pregnancy outcome was 36.1 weeks (30.7-39.0) in fetuses with DA constriction vs. 33 weeks (23.3-37.3) in fetuses who did not receive NSAIDs or achieve DA constriction (p=0.040). Eleven of 12 patients (92%) with DA constriction survived to live-birth, whereas 4/9 patients (44%) who did not receive NSAIDs or achieve DA constriction survived (p=0.046). In conclusion, our findings demonstrate the proof of concept that NSAIDs mitigate circular shunt physiology by DA constriction and improve PR among fetuses with severe EA/TVD. Although the early results are encouraging, further investigation is necessary to determine safety and efficacy.