When an embryo in utero it is going through a rapid stage of growth and development, particularly in the brain and nervous system. Being exposed to hormones, such as stress hormones, or other terotogens such as chemical drugs or alcohol can negatively affect the embryo's development.
Stress during pregnancy, much like exposure to terotogens, may cause premature birth, and low birth weights. At birth children who have been exposed to high levels of stress may need assistance breathing as they may have under developed lungs due to low birth weight. Preterm babies are also more likely to experience developmental delays later in childhood. In addicting to cognitive and physical development there is research to suggest that exposure to high levels of stress hormones in utero may affect the temperament of the child after birth, meaning that they may be more likely to be irritable or be predisposed to depression.
The problem is there is no hard and fast rule regarding stress during pregnancy as there is with the use of substances as the amount of stress that is bad for the health varies from person to person. Unlike exposure to terotogens exposure to stress hormones will not result in withdrawal symptoms for the infant.