The seasonal occurrence of births of children with ventricular septal defects (V.S.D.) was examined for a series of 302 cases from New England. The overall series showed a moderate peak in the summer, which was entirely attributable to a strong tendency for complex V.S.D. to occur in summer. Complex V.S.D. occurred 4.4 times more frequently in urban counties than rural counties, and the seasonal trend was strongest in urban areas. The seasonal peak was not associated with birth-weight, gestational age, maternal age, birth order, or sex, but a significant secular decline was noted over the five study years. One subgroup of complex V.S.D.—those with associated atrial septal defects—showed no evidence of seasonal clustering. V.S.D. associated with persistent ductus arteriosus, a defect previously related to viral teratogens, occurred almost exclusively during a brief portion of the year. The data suggest that simple and complex V.S.D. are ætiologically distinct, and that V.S.D. with associated cardiac anomalies, in particular persistent ductus arteriosus, is caused by an environmental agent more prevalent in early winter and in cities.
