The four most common teratogens are tobacco, alcohol, medications and illegal drugs. Teratogens are substances that could harm a developing fetus by affecting growth and causing birth defects or death.
The use of tobacco during pregnancy is dangerous because the smoke causes the blood vessels to constrict. This means that the fetus receives less blood supply and nutrients, restricting the fetus's growth and potentially leading to an underweight baby or stillbirth.
Excessive alcohol consumption during early pregnancy can result in premature labor or miscarriage. Regular alcohol consumption throughout pregnancy could lead to babies who are born with physical and mental ailments.
Both over-the-counter and prescription medications have the potential to cause problems with the development of a fetus. The use of illegal drugs may cause underweight birth, placental abruption, developmental delay, addiction in the baby and death. Some illegal drugs could also cause brain, liver or kidney damage to the baby.