Teratogens are substances that can introduce developmental abnormalities to the embryo and/or the fetus.
A common example of teratogens are the TORCH infections(Toxoplasmosis, Rubella, Cytomegalovirus, Herpes Simplex, and others such as syphilis, parvovirus, and varicella-zoster). Another example is alcohol(it can cause Fetal Alchohol Syndrome).