Directly after birth, the baby starts to breathe by itself and its blood needs to be oxygenated by its lungs. All the blood pumped out of the right ventricle (2) into the pulmonary artery (3) should now go to the lungs. The bypass created by the duct is no longer needed, and it starts to close and then shrivel up.
In babies born full term, the blood usually stops flowing via the duct during the first 24 hours after birth. Eventually, the duct withers away entirely.