With the first breath of life, pulmonary aerial resistance drops dramatically. This is due to the oxygenation of the lungs causing vasodilatation of the pulmonary vessels. Clamping of the umbilical cord doubles peripheral resistance and causes an increase in aerial blood pressure. As soon as right atrial, pressure drops below left atrial pressure, the foramen ovale will close (valve-like mechanism), establishing the adult-type blood circulation. The rising systemic and falling pulmonary aery pressure causes a flow reversal through the ductus aeriosus from right to left to left to right within minutes of bih. Complete closure of the ductus aeriosus appears to be due to a decline of local prostacyclin levels and usually occurs within 24 to 48 hours after bih. Ref: Moon D.A., Foreman K.B., Albeine K.H. (2011). Chapter 4. Hea. In D.A. Moon, K.B. Foreman, K.H. Albeine (Eds), The Big Picture: Gross Anatomy.