Teratogens are substances that may cause an abnormality following fetal exposure to the teratogen during pregnancy. The first half of pregnancy is the most vulnerable period to teratogens. The types or severity of abnormalities caused by a teratogenic agent depends on the genetic susceptibilities carried by the mother and the fetus.

Known teratogens include:

• Cyclopamine
• Lithium
• Thalidomide
• Valproic acid
• Vitamin A