A teratogen is any substance or factor that can cause malformation or abnormal development in a fetus during pregnancy. Common teratogens include certain medications, alcohol, and infections. Exposure to these agents can lead to birth defects or developmental issues in the child. The effects of teratogens depend on various factors, including the timing of exposure, the dose, and the genetic susceptibility of the fetus. For example, exposure to thalidomide during the first trimester can result in limb deformities, while rubella infection can lead to serious heart defects. Understanding teratogens is crucial for prenatal care and maternal health.