Teratology is the scientific study of causes and mechanisms of malformation during the human development. Fetal diseases, mechanical effects and retarded development of the embryo and the fetus are some of the causes of CDDs (congenital developmental disorders) according to various studies. Both mystical and scientific theories were developed in the past to explain the origin of Teratology; some theories stating that it originated from the position of the stars, maternal impressions, hybridization, and oligohy dramnios, among others. Today, biological assumptions on abnormalities seem to have more weight than the unproven theories given in the past. Scientific studies have revealed that the real causes of congenital developmental disorders include: mechanical effects, biological factors, physical factors and chemical substances (Ujhazy, Mach, Navarova, Brucknerova, & Dubovicky, 2012). Fig. 1. 1. The Birth of Modern Teratology (McCormick, 2012)

The contemporary science of teratology started in the 1930s with the release of a study that was done on expectant pigs. During the experiment, the pigs were given food lacking vitamin A. The results showed malformed piglets, especially lack of eyes, leading to a summary that, lack of the vitamins is central to the poor development of the body parts like the eyes. The father of experimental teratology is the physicist Josef Warkany (Ujhazy, Mach, Navarova, Brucknerova, & Dubovicky, 2012). He was the first to provide evidence-based research that congenital developmental disorders can be induced in mammals, through his experiments in the 1930s and 40s. This led to the definition of both the genetically and environmentally provoked structural defects. Congeners of biologically active molecules, e.g. amino acid mimicking azaserine, were used experimentally with animals to show the vulnerability of mammalian fetus and embryos to xenobiotic poisons. Consequently, amino-protein was utilized in the 1950s on a human embryo to induce abortion. Low oxygen concentration, changes in temperature, radiation, hormones such as cortisone, androgens, estrogens, hypervitaminosis and hypovitaminosis are some of the physical factors that were utilized in further experiments. A variety of chemicals and

Experiments with animals led to a report that was published during the late 40s, revealing the environmental and genetic effects and their related combinations, as some of the causes of defects and other malformations in animals (Ujhazy, Mach, Navarova, Brucknerova, & Dubovicky, 2012).
Describe the effects of various teratogens during different periods of development

About 7% of congenital defects are of teratogenic origins. Fetal morphology or its subsequent functions can be altered by exposure to a physical factor, infectious agent, chemical, or deficiency. The crossing of these agents into the placenta results in teratogenicity. Substances with high molecular weight like the heparin, cannot cross the placenta, and hence not considered teratogenic. During the embryonic development, the embryos are most vulnerable to teratogens when the rapid differentiation phases occur. This is also the most crucial stage of the early development or growth of any organ as illustrated for each organ in the picture below. For instance, the growth and development of the brain are from week three to week sixteen, even though its differentiation stretches into infancy (Chung, n.d.).

Any agent that is capable of provoking or hastening congenital malformation is known as a teratogen. The knowledge of these human teratogens helps in preventing one from its exposure that may lead to congenital malformations during the human development. The teratogenicity of drugs, food additives, and pesticides are tested to reduce exposure of expectant women to these agents (Chung, n.d.).

Fig. 1. 2. Effects of teratogens at different stages of development (HoRC, n.d.)Describe the Principles of Teratology

1. To Characterize teratogenic exposures, one must consider, the particular agent, genetic susceptibility, the dose of the agent, and the development phase (Can, 2007).

2. Both general and specific effects like morphogenesis alterations, death, and carcinogenesis and specific syndromes, magnitude of risk, and prenatal diagnosis, respectively, are used to characterize teratogenic effects (invasive and non-invasive techniques) (Can, 2007).

3. The United States Food and Drug Administration (A, B, C, D, X) developed 5 categories that manufacturers have to adhere to and label appropriately on their pregnancy medication to minimize fetal exposure to teratogens (Can, 2007).

4. The placenta can be crossed by any substance given to the mother, whose molecular size is small enough to pass through the placenta, or it's not destroyed during the placental passage. The transplacental transfer starts at the fifth week of embryo development. The passage of substances with low molecular weight depends on concentration…