A pregnant woman was recently arrested for using heroin. She is addicted to heroin, and there is a risk that her fetus will be born a heroin addict as well. Heroin is a for the fetus.

Heroin is a "teratogen" for the fetus.        

Explanation:

A teratogen is an agent or substance capable of provoking malformations to the embryo during pregnancy. These are chemicals that affect the fetus´s normal development, resulting in anomalies.  The word proceeds from the latin "teratos" which means "monster", referring to anatomical malformations.

Many substances may act as teratogens, such as illegal drugs, alcohol, pills, among others.

Heroin is a teratogen drug, that is capable of going through the placenta and reach directly the fetus. There are many risks to the baby, like being born with abstinence symptoms such as shakes, vomits, diarrhea, etc. They might be premature, showing cardiac problems, and behavior or learning problems.

The harm to the baby depends on the quantities and concentration of the drug while consuming it, pregnancy stage, and how much time the drug acts on the baby.