Which fetal structure becomes the fossa ovalis after birth?

• A. Foramen ovale
• B. Ductus arteriosus
• C. Ductus venosus
• D. Umbilical vein

Answer: (A)

The fossa ovalis is the remnant of the foramen ovale, a fetal opening in the interatrial septum that lets blood flow from the right atrium to the left atrium, bypassing the nonfunctional lungs. After birth, breathing expands the lungs and lowers pulmonary resistance, while left atrial pressure rises. This pressure shift functionally closes the foramen ovale, and over time the tissue fuses to leave a thin, oval-shaped depression—the fossa ovalis.

Other fetal shunts don’t form the fossa ovalis: the ductus arteriosus becomes the ligamentum arteriosum, the ductus venosus becomes the ligamentum venosum, and the umbilical vein becomes the ligamentum teres (round ligament) of the liver.