Why Efforts to Expand the Meaning of "Teratogen" Are Unacceptable Disagreement about nomenclature in teratology is not new. Dissent even about the very fabric of the discipline-what congenital malformations consist of-has often been voiced. Time, instead of resolving such diffi culties, has sometimes worsened them. For example, in the past it was agreed that congenital malforma tions are abnormalities of structure present at birth, but differences of opinion concerning where the line between normal and abnormal was to be drawn prevailed. It was obvious that, in order to discover the causes of congenital malformations and cast strategies for their prevention, it would be necessary to have knowledge of the baseline of their frequency, and that this required uniformity of definition of terms. Since malfor mations of primary social concern are those having grave outcomes (and are, paradoxically, also the commonest ones), it is logical that such condi tions were the first consideration of investigators and were the defects whose frequency was considered to comprise the required baseline.