A fetotoxin is a substance or agent that can poison or cause degenerative effects in a developing fetus or embryo. The effects are generally similar to those an adult would suffer from exposure. This is closely related to a teratogen, which causes malformations of an embryo or fetus. Both fetotoxins and teratogens are reproductive toxins, substances which cause damage to one's reproductive and/or endocrine system and/or a developing fetus.