Sir.—Thibeault and his colleagues (Am J Dis Child 131:741-745, 1977) have offered clinical evidence to support the hypothesis of "accelerated maturation" of the ductus arteriosus in infants with intrauterine growth retardation. Unfortunately, no histological correlation was described in the four infants considered to have a functionally closed ductus arteriosus who died and had a postmortem examination. The authors mention that the ductus arteriosus in each of these cases appeared to be markedly constricted although probe patent. The use of a probe to determine patency and degree of constriction of the ductus is extremely inaccurate. Often, patency is primarily a function of the autopsy surgeon's strength. In addition, thrusting a probe through the lumen of the ductus only serves to destroy the intimal thickening described as "cushions," which are believed by many authors1 to contribute to closure of the ductus arteriosus. If useful information concerning the maturation of the ductus