A teratogen is defined as a substance that can __________.

A teratogen is specifically defined as a substance that crosses the placental barrier and has the potential to cause harm to an unborn child during prenatal development. This may include a variety of environmental factors, medications, drugs, infections, or even certain maternal health conditions that can adversely affect fetal growth and development, leading to birth defects or other developmental problems.

The concept of teratogens is critical in the study of prenatal health, as it emphasizes the impact that external substances can have during the crucial stages of fetal development. Identifying and understanding teratogens helps in advising pregnant individuals regarding behaviors and substances to avoid to promote the healthy development of their child. In contrast, the other options refer to factors that either promote health or do not directly relate to the risks posed by teratogenic substances.