Correct! Most CHDs are multifactorial, having both genetic and environmental factors.

Atrial septal defects (ASDs) are relatively common anomalies, occurring in approximately 10% of cardiac defects. Label the following structures involved in ASDs.

List the steps of atrial septation, starting from the septum primum and ending with foramen ovale.

(Write your answer down! We will review this at the end)

Several causes of ASD exist, including:

• Foramen primum defect - the leading edge of septum primum failed to fuse with the endocardial cushions; defect is located low in the interatrial septum

• Foramen secundum defect - excessive resorption of septum primum when forming the foramen secundum may leave an inadequate valve to cover foramen ovale (or hypoplastic growth of the septum secundum leaves a foramen ovale too large to be covered by the normal-sized valve); defect is centrally-located within the interatrial septum

• Sinus venosus defect - a hole is formed within the interatrial septum cranial to foramen ovale caused by abnormal incorporation of the right horn of sinus venosus; defect is located cranial to foramen ovale

• Common atrium - lack of septation

After reviewing the ultrasound image again, you determine that although a septum has formed, a defect has developed in the middle of the interatrial septum. Which type of ASD do you suspect?