A rare congenital heart defect, in which the left side of the heart doesn't develop fully and is too small.

Hypoplastic left heart syndrome is a heart condition present at birth. That means it's a congenital heart defect. With hypoplastic left heart syndrome, the left side of the heart can't pump blood well. Instead, the right side of the heart must pump blood to the lungs and to the rest of the body.

Symptoms of hypoplastic left heart syndrome in babies include changes in skin, lip or fingernail color, rapid and difficult breathing, poor feeding, cold hands and feet, a weak pulse, and being more drowsy or inactive than is typical.

Treatment for hypoplastic left heart syndrome may include medicines, heart surgery or a heart transplant. Advances in care have improved the outlook for babies born with the condition.