SEASONAL cycles have long been analysed to study economic trends, but only within the past two decades has seasonal analysis been applied to congenital malformations. Since McKeown and Record (1951) carefully studied large numbers of anencephalic stillbirths and found a seasonal pattern of a winter excess and a summer trough, a steady output of articles dealing with the seasonality of malformations has appeared. The scope of malformations analysed for seasonal trend has been somewhat narrow,

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... the numbers of cases have been small. The malformations most often studied for seasonal trend include anencephaly, spina bifida, clefts of the lip and palate, congenital dislocation of the hip, Down's syndrome, hydrocephaly, and patent ductus arteriosus. Bailar and Gurian (1965) systematically reviewed past research on seasonal incidence of malformations.