The ability of a drug to produce structural deformations and functional abnormalities in the developing embryo or the fetus is called teratogenicity, and the drug producing this effect is known as a teratogen. Teratogenic effects include stillbirth, miscarriage, intrauterine growth restriction, and neurocognitive delay. A teratogen may affect the embryo at different stages of development, which is important in determining the type and extent of the damage. During blastocyst formation, the early embryonic development stage, teratogens can kill some of the cells in the blastocyst, resulting in the death of the embryo. Organogenesis is a well-defined sequence of events during embryonic development, where the cells of the embryo differentiate and organize themselves into distinct organs and organ systems.. Exposure to the teratogen at a particular time period produces specific organ malfunction. During histogenesis and functional maturation of the fetus, teratogens interfere with the pregnant woman's nutrient supply or hormonal balance. This indirectly results in functional and developmental abnormalities. For example, angiotensin II has an important role in renal function and the development of the fetus. Angiotensin-converting enzyme (ACE) inhibitors and angiotensin receptor antagonists cause fetal malformation when administered in the early stages of pregnancy as well as oligohydramnios and renal failure in the later stages.