Why can exposure to teratogens during the first trimester be especially harmful?

Exposure to teratogens during the first trimester is especially harmful because this period encompasses the primary stages of organogenesis, the tightly programmed process during which the embryo’s organ systems are formed.1

Because each organ system has a specific, non-repeating critical period for development, exposure to harmful agents during these windows is highly likely to cause structural malformations. During the embryonic stages (the first eight weeks after conception), the embryo is particularly susceptible to chemical-induced cell death; if this death occurs in cells undergoing active cell division, the corresponding organs will be affected. While the outcome of such exposure can range from functional central nervous system abnormalities to prenatal growth restriction, severe instances may result in miscarriage or fetal death.12

It is a fundamental ethical principle to protect the health and safety of the developing fetus by minimizing exposure to known teratogens, such as certain drugs, alcohol, and environmental toxins, especially during these critical early stages of human development.21