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It has been scientifically proven that the human fetus is relatively susceptible to impact from adverse conditions within the mother’s environment. Sub-par fetal conditions often cause various degrees of developmental delays, both physical and mental, for the growing baby. Although some variables do occur as a result of genetic conditions pertaining to the father, a great many are directly brought about from environmental toxins that the mother is exposed to.

Introduction
Historically, it was recognized that certain substances that were harmful to the pregnant woman were often as harmful,if not more harmful, to the fetus as well. In the last century evidence has emerged that even environmental exposures that are not overtly harmful to the mother herself could cause serious harm to the developing fetus. Substances that caused birth defects and other adverse effects to the fetus, in the absence of recognized toxicity to mothers, were: the estrogenic hormone diethylstilbestrol (often taken by mothers during pregnancy and found to cause genital abnormalities); thalidomide (taken to control nausea during pregnancy and found to cause limb abnormalities); and methylmercury (a neorotoxin found in contaminated fish. In addition, more recent research has shown evidence of the adverse effects associated with the consumption of ethanol during pregnancyMore recently, maternal tobacco smoking and even environmental tobacco smoke, modest consumption of ethanol during pregnancy, and low levels of exposure to lead and mercury have been found to be have adverse impacts on fetal growth and development.

Fetal Development
The prenatal environment is highly susceptible to toxicities found in the mother’s world. Recent research indicates that anywhere from one thousand to three thousand new chemicals have been introduced into the environment every year for over twenty years (Tortorello, 2012). These chemicals all have the potential to cause significant damage to a developing fetus, should they be introduced into the neonatal environment. Although the majority of these chemicals are known to cause illness in individuals of any age ( such as cigarette smoke, carbon monoxide, or lead) some are found in everyday items and supplies that one would not think twice about having around the house. In fact, many such items as lipstick, fragrant shampoo, and plastic food containers all have the potential to contain harmful chemicals that can cause serious obstacles for fetal development (Tortorello, 2012). In fact, studies have shown certain brands of lipstick and other cosmetic products to contain a toxic chemical known as ethylhexyl methoxycinnamate, an endocrine disrupter which causes interference with the body’s ability to make and secrete chemicals important for proper growth and development (Tortorello, 2012). When unknowing mothers accidently ingest this makeup it is transferred into the neonatal environment where it can directly impair fetal growth. Despite policies put in place for the specific purpose of regulating chemicals currently on the market, safety data only exists for about fifteen percent of newly discovered chemicals, leaving tens of thousands of older chemicals unregulated and unrecorded (Tortorello, 2012). When such unregulated chemicals find their way into the fetal environment, they have a variety of adverse effects on development and future functioning. When the human fetus experiences negative effects while in the prenatal environment, it is said to have experienced a “developmental toxicity” (Pohl et al., 1998). Developmental toxicities can result in structural abnormalities, altered growth, functional deficiencies, congenital neoplasia, or even death for the fetus (Pohl et al., 1998).

Placenta and its role in fetal development
Perhaps one of the most crucial components of the developmental process is the placenta. The placenta is a semipermeable membrane that allows certain substances to travel through to the fetus (ATSDR, 2012). The placenta is crucial to the well-being of the fetus, for it is the organ that mediates the exchange of blood and nutrients from the mother (Rahmalia, 2012). The primary functions of the placenta are to transport nutrients, oxygen, immunoglobulins, and hormones to the fetus, facilitate the elimination of waste products, perform endocrine functions, and prevent the mother’s immune system from attacking the fetus (Rahmalia, 2012).

The healthy placenta does form a barrier for most pathogens and for certain xenobiotic substances. However, it is by design an imperfect barrier since it must transport substances required for growth and development. Placental transport can be by passive diffusion for smaller molecules that are lipid soluble or by active transport for substances that are larger and/or electrically charged. Some toxic chemicals may be actively transported. The dose of a substance received by the fetus is determined by the amount of the substance transported across the placenta as well as the rate of metabolism and elimination of the substance. As the fetus has an immature metabolism, it is unable to detoxify substances very efficiently; and as the placenta plays such an important role in substance exchange between the mother and the fetus, it goes without saying that any toxic substances that the mother is exposed to are transported to the fetus, where they can then affect development. Carbon-dioxide, lead, ethanol (alcohol), and cigarette smoke in particular are all substances that have a high likelihood of placental transferal (ATSDR, 2012).

Identifying potential hazards for fetal development requires a basis of scientific information. In 2004, Brent proposed a set of criteria for identifying causes of congenital malformations that also are applicable to developmental toxicity in general. Those criteria are: relationship between particular effects and exposure to the substance. exposure and the specific effect. exposures and particular effects.
 * Well-conducted epidemiology studies consistently show a
 * Data trends support a relationship between changing levels of
 * Animal studies provide evidence of the correlation between substance

Fetal effects from maternal exposures
As previously mentioned, conditions the mother is exposed to during pregnancy have the potential to damage to the developing fetus (ATSDR, 2012). Even exposure to seemingly harmless levels of substances can cause toxins to be stored within the mother’s body, where they are then transferred to the fetus via the placenta and cause damage to the delicate structures (ATSDR, 2012). Substances which have been found to be particularly harmful are lead (which is stored in the mother’s bones), cigarette smoke, mercury (a neurological toxicant consumed through fish), carbon dioxide, and ionizing radiation (ATSDR,2012). Factors such as heat and noise have also been found to have significant effects on development (ATSDR, 2012).

Individual substances and their impacts on development
Cigarette smoke—stillbirth, placental disruption, prematurity, lower mean birth weight, physical birth defects (cleft palate etc.), decrements in lung function, increased risk of infant mortality (ATSDR,2012). Carbon dioxide--- decreased oxygen delivery to brain, intellectual deficiencies (ATSDR, 2012) Lead---miscarriage, low birth weight, neurological delays, anemia, encephalopathy, paralysis, blindness, (ATSDR, 2012, Lanphear, et al., 2005) Mercury---cerebral palsy, limb defects, mental retardation (Lanphear, et al., 2005). Ionizing radiation---miscarriage, low birth weight, physical birth defects, childhood cancers (ATSDR,2012)

Dioxin Exposure
One particular substance of recent concern is dioxin. Dioxin is a toxic chlorine based compound that can be found everywhere in the environment (Schneider, 1994). Originally a byproduct of the incineration of chlorine-based chemicals (usually during manufacturing), dioxin falls to earth from factory smoke stacks, where it is consumed by various animals (Schneider, 1994). When intoxicated animals are consumed by expecting mothers, the chemical is released into their system, where it then transfers to the fetus (Schneider, 1994). Dioxin exposure during fetal development has been found to cause increases in several different types of cancer, reproductive dysfunction, growth problems, and thwarted cell growth (Schneider, 1994). Further research is still being conducted to investigate the full repercussions of prenatal dioxin exposure (Schneider, 1994).

Air pollution
Air pollution represents another source of detriment to growth and development. Compounds such as carbon monoxide, sulfur dioxide and nitrogen dioxide all have the potential to cause serious damage when inhaled by an expecting mother (Le et al., 2012). Low birth weight, preterm birth, intra-uterine growth retardation, and congenital abnormalities have all been found to be associated with fetal exposure to air pollution (Minguillon, 2012). Although pollution can be found virtually everywhere, there are specific sources that have been known to release toxic substances and should be avoided if possible by those who wish to remain relatively free of toxicants. These substances include, but are not limited to: steel mills, waste/water treatment plants, sewage incinerators, automotive fabrication plants, oil refineries, and chemical manufacturing plants (Le et al., 2012). Expecting mothers should make efforts to avoid exposure to air pollution at all costs.

Pesticides
A final area of concern to those wishing to avoid prenatal exposure to toxicants would be pesticides. Created for the specific purpose of causing harm (to insects, rodents, and other pests), pesticides have the potential to serious damages to a developing fetus, should they be introduced into the fetal environment. Studies have shown that pesticides, particularly fungicides, have shown up in analyses of infant’s cord blood, proving that such toxins are indeed transferred into the baby’s body (Wickerham et al., 2012). Overall, the two pesticides most frequently detected in cord blood are diethyltolaumide ( a commonly used repellant) and vinclozolin (a fungicide) (Wickerham et al., 2012). Although pesticide toxicity is not as frequently mentioned as some of the other methods of environmental toxicity, such as air pollution, contamination can occur at any time from merely engaging in everyday activities such as walking down a pathway near a contaminated area, or eating foods that have not been washed properly (Wickerham et al., 2012). IN 2007 alone, 1.1 billion pounds of pesticides were found present in the environment, causing pesticide exposure to gain notoriety as a new cause of caution to those wishing to preserve their health and that of the unborn (Wickerham et al., 2012).

Conclusion
As there are a variety of substances that can prove harmful to a developing fetus, it is crucial that individuals who are expecting take extra precautions to limit exposure to toxicity. Although it is impossible to completely avoid all sources of contamination, by being careful and conscious of one's surroundings, it is possible to significantly decrease harmful exposure. Research is still being conducted on various substances to discover the full extent of teratogens that are found in the environment and around everyday communities.

References Centers for Disease Control and Prevention, Agency for Toxic Substances And Disease Registry (2012). Principles of pediatric environmental health the child as susceptible host: A developmental approach to pediatric environmental medicine (course:WB 2089). Retrieved from http://www.atsdr.cdc.gov/csem/csem.asp?csem=27&po=8

Lanphear, B. P., Vorhees, C. V, Bellinger, D.C. (2005). Protecting children from environmental toxins. PLOS Medicine 2(3). Retrieved from http://www.plosmedicine.org/article/info%3Adoi%2F10.1371%2Fjournal.pmed.0020061

Le, H.Q., Batterman, S.A., Wirth, J.J., Wahl, R. L., Hoggat, K.J., Sadeghnejad, A., Hultin, M.L. & Depa, M. (2012). Air pollution exposure and preterm and term small-for-gestational-age births in Detroit, Michigan:Long-term trends and associations. Environmental International, 44, 7-17.

Minguillon, M.C., Schembari, A., Triguero-Mas, M., de Nazelle, A., Dadvand, P., Figueras, F., Salvado, J.A., Grimalt, J.O., Nieuwenhuijsen, M., & Querol, X. (2012). Source apportionment of indoor, outdoor and personal PM2.5 exposure of pregnant women in Barcelona, Spain. Atmospheric Environment, 59, 426-436.

Pohl, H.R., Smith-Simon, C. & Hicks, H. (1998). Health effects classification and its role in the derivation of minimal risk levels: Developmental Effects. Regulatory toxicology and pharmacology, 28, 55-60.

Rahmalia, A., Giorgis-Allemand, L., Lepeule, J., Philippat, C., Galineau, J., Hulin, A., Charles,M., Slama, R., & The Eden Mother-Child Cohort Study Group (2012). Pregnancy exposure to atmospheric pollutants and placental weight: An approach relying on a dispersion model. Environmental International, 48, 47-55.

Schneider, K. (1994, May 11). Fetal harm is cited as primary hazard in dioxin exposure. New York Times. Retrieved online from http://www.nytimes.com/1994/05/11/us/fetal-harm-is-cited-as-primary-hazard-in-dioxin-exposure.html?pagewanted=all&src=pm

Tortorello, M. (2012, March 14). Is it safe to play yet-Going to extreme lengths to purge household toxins. New York Times. Retrieved online from http://www.nytimes.com/2012/03/15/garden/going-to-extreme-lengths-to-purge-household-toxins.html?pagewanted=all

Wickerham, E.L., Lozoff, B., Shao, J., Kaciroti, N., Xia, Y. & Meeker, J.D. (2012). Reduced birth weight in relation to pesticide mixtures detected in cord blood of full-term infants. Environmental International, 47, 80-85.