Final answer:
Teratogens are harmful environmental agents affecting fetal development during pregnancy.
Explanation:
Teratogens are environmental agents that can negatively affect the growth and development of an unborn child. These agents include substances like alcohol, drugs, and pollutants that can harm the fetus when exposed during pregnancy, especially in sensitive phases of development. The most vulnerable period for fetal organs is early in pregnancy, before the mother may even know she is pregnant.
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