Teratogenicity is the ability of a drug to produce structural malformations and functional abnormalities in the developing embryo or the fetus. Teratogenic effects include stillbirth, miscarriage, intrauterine growth restriction, and neurocognitive delay. Teratogens are selective in action and exert their effects at specific stages of fetal development. During blastocyst formation, teratogens can inhibit cell division, and kill the embryo. Exposure to certain teratogens like thalidomide during organogenesis—a sequential process of organ formation in the embryo—produces specific organ malfunction. During histogenesis and functional maturation, certain teratogens may interfere with oxygen and nutrient supply to the fetus. This affects the growth and development of the fetus. Some teratogens directly affect the differentiation process in developing tissues and lead to skeletal deformities.