A teratogen is anything that can disrupt the development of an embryo or fetus in a pregnant mother's womb. A teratogen may cause a birth defect, malformation, or terminate the pregnancy altogether.
It can be in the form of harmful drugs or chemicals, like birth control pills, nicotine, alcohol, or some types of medication. Exposure to infectious agents like German Measles or Chicken Pox has also been found to have teratogenic effects. Physical factors such as exposure to x-rays or increased body temperature from taking hot baths may also have negative effects on the developing fetus. Certain medical conditions of the mother like diabetes or hypothyroidism may also lead to defects or fetal death.