The incidence of a transient murmur resembling that of pulmonary arterial stenosis in healthy premature infants during the first six weeks of life was 67% (ten of 15) whereas only 5% of term infants (three of 61) were noted to have such a murmur during the few days spent in hospital. None of 25 term infants examined at home during the first four weeks of life had the murmur.
The murmur was most frequently seen in babies with a birth weight below 2,200 gm (5 lb).
Hypoplasia of the pulmonary artery branches or unusual alignment of the main pulmonary artery with its branches in premature infants are postulated as causes of the murmur which disappears as the infant grows.
Lisa M. Dunkle, Richard D. Rowe. Transient Murmur Simulating Pulmonary Artery Stenosis in Premature Infants. Am J Dis Child. 1972;124(5):666–670. doi:10.1001/archpedi.1972.02110170044006