1. An agent or factor which causes malformation of a developing embryo.
2. An environmental agent that can cause abnormalities in a developing organism resulting in either fetal death or congenital abnormality.

Normally a human fetus is separated from the mother by the placental barrier, but if the barrier is imperfect and permits a number of chemical and infectious agents to pass to the fetus, then abnormalities may result.

1. An agent that interrupts or alters the normal development of a fetus, with results that are evident at birth; for example, such agents as, a chemical, virus, or ionizing radiation.
2. Any agent that can disturb the development of an embryo or fetus.

Teratogens may cause a birth defect in the child or it may halt the pregnancy outright. The classes of teratogens include radiation, maternal infections, chemicals, and drugs.