A primitive but still active belief is that pregnant women may be suggested in such a way that they give birth to monstrous children.

Malebranche theorized that monstrous births were caused by the imagination of the mother on the foetus, which was achieved through communication between the nerves of the mother and the nerves of the foetus. 

For example, a monstrous birth might result if a pregnant woman saw a particularly ugly or frightening object. Malebranche based this on the 16th century belief that imagination had the power to affect the outside world.

Blondel’s text was the first scientific work to refute the role of the imagination of pregnant women in creating monstrous births.  More generally, although this belief remained popular, it was gradually rejected from aesthetics, natural philosophy, and medicine in the eighteenth century.

Blondel used the regularity of nature and its mechanical functioning as a ground against his opponents. He illustrated this with numerous clinical examples. Blondel also rejected the  “imaginative” theory of monstrous births on the basis that belief in the power of the imagination was an unscientific idea held by lay people.

However, it is still a belief among most people that imagination can influence teh birth of a child. In the Boston Medical and Surgical Journal, p.98 (1839) there is an article about a snake-man “caused by strong maternal impression (q.v.) during pregnancy of a woman attacked by a rattle-snake… Body had shape and action of a snake, and serpent-like teeth.