Teratology is the study of serious deviations from normal growth and development in organisms. The word teratogenesis is derived from the Greek gennan, meaning to produce, and terata, meaning monster. In humans, teratology is the study of chemicals, drugs, medications, alcohol, disease, or other environmental agents in relation to fetal abnormalities and birth defects. To be classified as a teratogen, an agent must cause either low birth weight, intrauterine growth retardation, or small size for gestational age; stillbirth or miscarriage; structural abnormalities; or functional defects (mental or developmental retardation). To cause birth defects, a teratogen must reach the developing fetus. Teratogens can also cause premature birth and fetal/newborn addiction to pharmacological agents. Chemicals such as thalidomide, tetracycline, valproic acid, cocaine, alcohol; metals such as lead and mercury; viruses such as syphilis, herpes, and rubella can all be regarded as teratogens.
Birth defects and abnormalities have occurred throughout history. Rock carvings and drawings depict individuals with deformities and malformations; writings from ancient Babylon describe them, as do clay tablets believed to date back to 2000 B.C. found in the library of an Assyrian king around 700 B.C..
Before the advent of the modern study of birth defects in the eighteenth century, congenital abnormalities were viewed with superstition or as supernatural--perhaps punishment from the gods or the work of the devil. The first person to articulate the theory of "developmental arrest" was William Harvey in 1651, when he observed that a cleft lip in a newborn infant closely resembled the normal condition of a fetus at a certain stage of development. Albrecht von Haller (1768) and Kaspar Friedrich Wolff (1759) followed a similar line of thinking. Modern studies show that teratogens act by disrupting the normal developmental pathways of a growing fetus. So a small disruption of cell signalling by a teratogenic chemical can alter the development of a morphological feature. A teratogen may also be a mutagen, a substance which increase the chance of a DNAmutation, which may lead to birth defects.
Approximately 3-5% of all infants worldwide are born with some form of birth defect. Approximately 5% of those appear to be due to teratogens, a small percentage of which are preventable through pre-pregnancy counselling and the assessment of risk factors in certain populations. In many countries, educational and informational organizations are working toward reducing the number teratogen-related birth defects. For example, recent evidence indicates a woman can reduce her chances of bearing a child with spina bifida, one of the most common types of birth defects, by 50% simply by consuming at least 400 micrograms of folic acid daily for least one month prior to conception.
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