The Fetal and Newborn Heart At birth, the umbilical cord is clamped and the baby no longer receives oxygen and nutrients from the mother. With the first breaths of life, the lungs begin to expand. As the lungs expand, the alveoli in the lungs are cleared of fluid. An increase in the baby's blood pressure and a significant reduction in the pulmonary pressures reduces the need for the ductus arteriosus to shunt blood. These changes promote the closure of the shunt. These changes increase the pressure in the left atrium of the heart, which decrease the pressure in the right atrium. The shift in pressure stimulates the foramen ovale to close. The closure of the ductus arteriosus and foramen ovale completes the transition of fetal circulation to newborn circulation.
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