The circulatory adjustments and myocardial biochemical changes at birth and in the neonatal period are modified in direct relation to the degree of prematurity. The muscular coat of the pulmonary arterioles develops late in gestation; the more premature the infant, the less muscular are the pulmonary arterioles at birth. The most notable consequence of this is that the difference between systemic and pulmonary resistance after birth is greater among premature than among normal infants. Shunting through a ductus arteriosus is often audible. Developmental biological factors in the ductus arteriosus and hypoxia, so common among premature infants, may be factors that contribute to the delay in closure of the ductus in premature infants. The
propensity of the ductus to close at around 41 weeks after conception is clinically recognized. Developmental changes in myocardial structure and biochemistry may influence the function of the left ventricle in response to stress such as volume overload associated with the left-to-right shunt through a patent ductus arteriosus.