Hypothesis: anencephaly and spina bifida are usually preventable by avoidance of a specific but unidentified substance present in certain potato tubers.

An epidemiological pattern for occurrence of anencephaly and spina bifida (ASB) malformations has emerged and an hypothesis based on this pattern is proposed. A greater incidence of anencephaly among winter births suggesting that infants conceived in spring and summer run higher risk of malformation had already been noted. Because of the seasonal coincidence an infective explanation was unsatisfactory; the hypothesis offered here is that the potato is the responsible food because it may become teratogenic as it ages and is exposed to light. A potato pathogen Synchytrium endobioticum a fungus responsible for potato wart disease has been associated with a geographical distribution similar to that of ASB. A similar case can be made for potato blight fungus as the pathogen. Therefore pathological and chemical changes in the potato are described and correlated with ASB incidence geographically. Similarly antibiotic products of the tuber which induce steroid alkaloids are outlined. The conjecture of a teratogenic role for a steroid alkaloid is plausibly advanced to explain the correlation between potato blight and ASB incidence. Epidemiological evidence presented includes geographical distribution of blight and ASB; association with occupational class and nutritional habits thereof; maternal diet; season of the year; urbanization; maternal age and parity; (i.e. family appetite) recurrence risks in sibships; twin data; secular trends; and sex ratio. Possible access routes of the teratogen are peracutaneous inhalation and oral.