Individual development begins with fertilization and extends into postnatal life at least until processes of growth give way to those of maintenance. Its progress and direction are determined by genetic and environmental factors acting singly and in combination. Developmental anomalies include all anatomical, physiological, or biochemical deviations arising during development, especially during organogenesis (including placentogenesis), irrespective of cause. This discussion is confined to events occurring in intrauterine life.
The incidence of such deviations is obscured by many factors; much material is excluded by unobserved losses of embryos before gestation becomes evident; by lack of necropsies in all remaining cases including aborted and ectopic material; by inability to distinguish clearly between abnormal and normal; by failure to include apparently minor aberrations, and finally, because maldevelopment, especially if purely physiological, may pass unrecognized.
National statistics tend to be derived from lethal or obvious defects and are inadequately supported by necropsy. They give