Teratology is the study of abnormalities of physiological development. It is often thought of as the study of human congenital abnormalities, but it is broader than that, taking into account other non-birth developmental stages, including puberty; and other organisms, including plants. The related term developmental toxicity includes all manifestations of abnormal development that are caused by environmental insult. These may include growth retardation, delayed mental development or other congenital disorders without any structural malformations.Teratogens are substances that may cause birth defects via a toxic effect on an embryo or fetus. Known teratogens include: thalidomide, mercury, alcohol, lead, and polychlorinated biphenyls (PCBs).