Certain of the malformations known as monstrosities hardly represent the human type.

These are the cases in which the foetus is represented in a formless mass of tissue, or there is absence of development of important parts such as the nervous system or there is more or less extensive duplication of the body.
There has always been a great deal of popular interest attached to the malformations owing to the part which maternal impressions are supposed to play in their production.

In this, some striking impression made on the pregnant woman is supposed to affect in a definite way the structure of the child.

The cases, for instance, in which a woman sees an accident involving a wound or a loss of an arm and the child at birth shows a malformation involving the same part.
There is no association between maternal impressions and malformations, although there have been many striking coincidences.
All malformations arise during the first six weeks of pregnancy known as the embryonic period, in which the development of the form of the child is taking place, and during which time there is little consciousness of pregnancy.

Maternal impressions are usually received at a later period, when the form of the child is complete and it is merely growing.