Presence of a teratogen in the body of mother during pregnancy is responsible for mal growth of child or incomplete growth which may lead to abortion, still birth or even a defective child.
Step-by-step explanation:
A teratogen is a substance that is known to cause defects in growth of child during the intrauterine growth period. Thalidomide was used as a morning sickness drug which relieved the vomiting nature of the would be mother. But the presence of thalidomide was responsible for less growth of limbs of the child. Alcohol in intrauterine life may cause Fetal Alcoholic Syndrome which damages the fetus' sense organs and neural cord.
The presence of a teratogen during the 1st trimester is more serious than in last trimester because the main formation of different organs and systems take place in the 1st trimester and maturity takes place in third trimester.