Immediately after birth, the ____ constricts, causing a change in blood pressure that forces the foramen ovale to close?

• A. Foramen arteriosum
• B. Ligamentum Teres
• C. Ductus arteriosus
• D. Ductus venosum

Answer: (D)

When a newborn takes its first breaths, the ductus arteriosus constricts in response to higher oxygen levels and falling prostaglandin levels. This reduces the fetal right-to-left shunt via the ductus and shifts blood flow toward the lungs. As more blood goes through the lungs, pulmonary venous return to the left atrium increases, raising left atrial pressure relative to the right atrium. That pressure change pushes the flap of the foramen ovale closed, stopping the interatrial shunt. Over time the foramen ovale becomes a fibrous structure. The ductus venosus closes later after cord clamping, but its closure isn’t what drives the foramen ovale’s closure.