The conception of a maternal impression rests on the belief that a powerful mental influence working on the mother’s mind may produce an impression, either general or definite, on the child she is carrying.
Maternal impression, according to a long-discredited medical theory, was a phenomenon that explained the existence of birth defects and congenital disorders. The theory stated that an emotional stimulus experienced by a pregnant woman could influence the development of the fetus. For example, it was sometimes supposed that the mother of the Elephant Man was frightened by an elephant during her pregnancy, thus "imprinting" the memory of the elephant onto the gestating fetus. Mental problems, such as schizophrenia and depression, were believed to be a manifestation of similar disordered feelings in the mother. For instance, a pregnant woman who experienced great sadness might imprint depressive tendencies onto the fetus in her uterus.
The theory of maternal impression was largely abandoned by the 20th century, with the development of modern genetic theory.
In folklore, Maternal Imprinting, or Versehen as it is usually called, is the belief that a sudden fear of some object or animal in a pregnant woman can cause her child to bear the mark of it. Perhaps the first and most famous instance of this is that of Jacob in the book of Genesis:
Jacob, then, left in charge of the rest of the flocks, did this. He took green branches of poplar, and almond, and plane, and partly peeled them; so that (now the bark had gone) the white shewed through where they had been stripped, whereas the parts he had left untouched remained green; everywhere the colour was varied. These branches he fitted into the troughs where his flocks were watered, so that when they came to drink, they should have these speckled branches before their eyes, and the dams would conceive in full view of them. Looking at these branches at the very heat of their coupling, the dams bore spotted and speckled, and piebald young.
Oswald Spengler understood Maternal Imprinting to be a folkloric understanding of what he called “blood feeling” or the formation of a group aesthetic of a bodily ideal:
What is called the “Versehen” of a pregnant woman is only a particular and not very important instance of the workings of a very deep and powerful formative principle inherent in all that is of the race side. It is a matter of common observation than elderly married people become strangely like one another, although probably Science with its measuring instruments would “prove” the exact opposite. It is impossible to exaggerate the formative power of this living pulse, this strong inward feeling for the perfection of one’s own type. The feeling for race-beauty — so opposite to the conscious taste of ripe urbans for intellectual-individual traits of beauty — is immensely strong in primitive men, and for that very reason never emerges into their consciousness. But such a feeling is race-forming. It undoubtedly molded the warrior- and hero-type of a nomad tribe more and definitely on one bodily ideal, so that it would have been quite unambiguous to speak of the race-figure of Romans or Ostrogoths.
Pliny also comments at length about the phenomenon of postpartum Maternal Impression in bears.