First-Trimester Detection of Conotruncal Anomalies Using Transabdominal SlowflowHD: A Case Series and Proof-of-Concept Study.

Background: Detection of conotruncal anomalies in the first trimester remains challenging. SlowflowHD, a Doppler modality with enhanced low-flow sensitivity, may improve visualization of cardiac outflow tracts.

Objective: To evaluate the utility of transabdominal SlowflowHD in detecting conotruncal congenital heart defects (CHDs) during routine first-trimester ultrasound screening.

Methods: Between January 2023 and April 2025, 1080 singleton pregnancies underwent ultrasound screening at 12 + 0 to 13 + 6 weeks' gestation using the Voluson Expert 22 with an RM7C probe. Cardiac evaluation included grayscale imaging and monochrome, non-directional SlowflowHD of the four-chamber view (4CV), left ventricular outflow tract (LVOT), and three-vessel trachea view (3VT). HDlive™ Flow was used to confirm flow direction when needed. All scans were transabdominal and performed by a single experienced operator. Suspected CHDs were referred for second-trimester echocardiography or genetic testing.

Results: Five fetuses were identified with outflow tract anomalies: tetralogy of Fallot (n = 3), double outlet right ventricle (n = 1), and transposition of the great arteries (n = 1). In all cases, SlowflowHD provided clearer delineation of septo-aortic continuity and vessel alignment than grayscale alone. Diagnoses were confirmed postnatally or during the second trimester; except in one case with trisomy 21, which was electively terminated.

Conclusion: SlowflowHD enhances early detection of conotruncal anomalies by improving visualization of key vascular structures. This modality may serve as a useful adjunct to conventional imaging in first-trimester screening. Further validation in larger cohorts is warranted.