Teratogens Teratogens Teratogen - external agents such as viruses, drugs, chemicals, and radiation that can cross the placental barrier and harm a developing embryo or fetus. The lack of a placental barrier allows most substances to pass from the mother's bloodstream to the fetus. The greatest effects of teratogens are on developing structures that are undergoing change. All periods of heightened sensitivity are referred to as sensitive periods. The most sensitive measures of teratogen effects usually are behavioral rather than structural.
Greater effects occur with greater doses of teratogens.
The effects of teratogens may not be obvious at birth. The long-term effects of birth defects depends on postnatal care. The same defect can be caused by different teratogens. A variety of defects can result from a single teratogen. Maternal Illnesses Genital Herpes -a sexually transmitted disease that can infect infants birth, causing blindness brain damage, or death.