Final answer:
The neonate's circulatory system changes are initiated by the clamping of the umbilical cord, leading to the closure of the foramen ovale and ductus arteriosus, caused by the first breath which decreases pulmonary blood pressure.
Step-by-step explanation:
The occurrence that initiates the changes in the neonate's circulatory system after birth is clamping of the umbilical cord. This process collapses the umbilical blood vessels, which leads to a series of circulatory adjustments. The first breath the newborn takes is crucial as it inflates the lungs, and decreases blood pressure in the pulmonary system. This change in pressure causes the closure of the foramen ovale and the ductus arteriosus, effectively redirecting blood flow to the lungs and liver, and reconfiguring the circulatory system from the fetal to the neonatal pattern. After the initial breath, blood flow increases into the pulmonary arteries, and this eventually leads to the permanent closure of the fetal shunts.