environmental exposure to perchlorate in women with hypothyroidism causes a significant risk of low iq in the child. avoiding relevant environmental toxins in pregnancy the american college of nurse-midwives recommends the following precautions to minimize exposure to relevant environmental toxins in pregnancy: avoiding paint supplies such as stained glass material, oil paints and ceramic glazes, and instead using watercolor or acrylic paints and glazes. checking the quality of the tap water or bottled water and changing water drinking habits if necessary. if living in a home built before 1978, checking whether lead paint has been used. if such is the case, paint that is crumbling or peeling should not be touched, a professional should remove the paint and the site should be avoided while the paint is removed or sanded. to decrease exposure to pesticides; washing all produce thoroughly, peeling the skin from fruits and vegetables or buying organic produce if possible. avoiding any cleaning supply labeled "toxic" or any product with a warning on the label, and instead trying natural products, baking soda, vinegar and/or water to clean. natural gas development in a rural colorado study of natural gas development, maternal residence within a 10-mile radius of natural gas wells was found to have a positive association to the prevalence of congenital heart defects (chds) and neural tube defects (ntds). along with this finding, a small association was found between mean birth weight and the density and proximity to the natural gas wells. maternal exposure through natural gas wells may come in the form of benzene, solvents, polycyclic aromatic hydrocarbons (pahs), and other air pollutants such as toluene, nitrogen dioxide, and sulfur dioxide. in pennsylvania, unconventional natural gas producing wells increased from zero in 2005 to 3689 in 2013. a 2016 study of 9384 mothers and 10946 neonates in the geisinger health system in pennsylvania found prenatal residential exposure to unconventional natural gas development activity was associated with preterm birth and physician-recorded high-risk pregnancy. in southwest pennsylvania, maternal proximity to unconventional gas drilling has been found to be associated with decreased birth weight. it was unclear which route of exposure: air, soil or water could be attributed to the association. further research and larger studies on this topic are needed. endocrine disruptors are compounds that can disrupt the normal development and normal hormone levels in humans. endocrine-disrupting chemicals (edcs) can interact with hormone receptors, as well as change hormone concentrations within the body, leading to incorrect hormone responses in the body as well as disrupt normal enzyme functioning. oil and gas extraction has been known to contribute to edcs in the environment, largely due to the high risk of ground and surface water contamination that comes with these extractions. in addition to water contamination, oil and gas extraction also lead to higher levels of air pollution, creating another route of exposure for these endocrine disruptors. this problem often goes under-reported, and therefore, the true magnitude of the impact is underestimated. in 2016, a study archived 2017-05-06 at the wayback machine was conducted to assess the need for an endocrine component to health assessments for drilling and extraction of oil and gas in densely populated areas. with the high potential for release of oil and gas chemicals with extraction, specifically chemicals that have been shown to disrupt normal hormone production and function, the authors highly emphasized the need for a component centering around endocrine function and overall health with health assessments, and how this in turn impacts the environment. role of the placenta the healthy placenta is a semipermeable membrane that 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 transferral. 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: well-conducted epidemiology studies consistently show a relationship between particular effects and exposure to the substance. data trends support a relationship between changing levels of exposure and the specific effect. animal studies provide evidence of the correlation between substance exposures and particular effects.