Truncus arteriosus is a congenital defect that occurs due to abnormal development of the foetal heart during the first 8 weeks of pregnancy.
The heart begins as a hollow tube, and the chambers, valves, and great arteries develop throughout the first 8 weeks of pregnancy.
The aorta and pulmonary artery start as a single blood vessel, which eventually divides and becomes two separate arteries.
Truncus arteriosus occurs when the single great vessel fails to separate completely, leaving a connection between the aorta and pulmonary artery.
This connection can be repaired surgically.