**2. Prevalence**

Congenital heart disease (CHD) accounts for around 1–1.2% of live births both in the United States and globally [1, 2]. AVSDs account for around 4–5% of all CHDs, with 5.38 cases per 10,000 live births, an increase from prior reports [1, 3]. It is the most common fetal cardiac anomaly detected on prenatal screening (**Figure 2**). Around half of the patients with AVSD have Down syndrome. However, approximately 45% of CHD patients with Down syndrome have AVSD [4, 5]. Most of these

#### **Figure 1.**

*Atrioventricular septum in the normal heart. The atrioventricular septum (AVS) lies between the right atrium (RA) and the left ventricle (LV). LA, left atrium; RV, right ventricle; MV, mitral valve; TV, tricuspid valve. "From: Cetta F, Truong D, Minich LL, Maleszewski JJ, O'Leary PW, Dearani JA & Burkhart HM. Chapter 29: Atrioventricular Septal Defects. In: Allen HD, editor. Moss & Adams' Heart Disease in Infants, Children, and Adolescents, Including the Fetus and Young Adult, 9th Edition. Philadelphia: Lippincott Williams & Wilkins, 2016; used with permission of Mayo Foundation for Medical Education and Research, all rights reserved."*

cases are isolated, although some may have pulmonary stenosis or atresia. There is an association with other anomalies like heterotaxy and Ellis-Van Creveld syndrome [6].
