Environmental toxicants and fetal development
Environmental toxins and fetal development is the impact of different toxins from the environment on the development of the fetus. This article deals with potential adverse effects of environmental toxins on the prenatal development of both the embryo or fetus, as well as pregnancy complications. The human embryo or 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.
Various toxins pose a significant hazard to fetuses during development. A 2011 study found that virtually all US pregnant women carry multiple chemicals, including some banned since the 1970s, in their bodies. Researchers detected polychlorinated biphenyls, organochlorine pesticides, perfluorinated compounds, phenols, polybrominated diphenyl ethers, phthalates, polycyclic aromatic hydrocarbons, perchlorate PBDEs, compounds used as flame retardants, and dichlorodiphenyltrichloroethane (DDT), a pesticide banned in the United States in 1972, in the bodies of 99 to 100 percent of the pregnant women they tested. Bisphenol A (BPA) was identified in 96 percent of the women surveyed. Several of the chemicals were at the same concentrations that have been associated with negative effects in children from other studies and it is thought that exposure to multiple chemicals can have a greater impact than exposure to only one substance.
- 1 Effects
- 2 Individual substances and toxin classes and their effects
- 3 Avoiding relevant environmental toxins in pregnancy
- 4 Role of the placenta
- 5 See also
- 6 References
- 7 Further reading
Environmental toxins can be described separately by what effects they have, such as structural abnormalities, altered growth, functional deficiencies, congenital neoplasia, or even death for the fetus.
One in ten US babies is born preterm and about 5% have low birth weight. Preterm birth, defined as birth at less than 37 weeks of gestation, is a major basis of infant mortality throughout childhood. Exposures to environmental toxins such as lead, tobacco smoke, and DDT have been linked with an increased risk for spontaneous abortion, low birth weight, or preterm birth.
Structural congenital abnormality
Toxic substances that are capable of causing structural congenital abnormalities can be termed teratogens. They are agents extrinsic to embryo or fetus which exert deleterious effects leading to increased risk of malformation, carcinogenesis, mutagenesis, altered function, deficient growth or pregnancy wastage. Teratogens are classified in four main categories:
- Drugs and chemicals. In addition to environmental chemicals, this category also includes recreational and pharamaceutical drugs in pregnancy.
- Vertically transmitted infections
- Radiation, such as X-rays
- Mechanical forces, such as oligohydramnios
Teratogens affect the fetus by various mechanism including:
- Interfering with cell proliferation rate, such as viral infection and ionization
- Altered biosynthetic pathways, as seen in chromosomal defects
- Abnormal cellular or tissue interactions, as seen in diabetes
- Extrinsic factors
- Threshold interaction of genes with environmental teratogens
Many cases of autism are related to particular geographic locations, implying that something in the environment is complementing an at-risk genotype to cause autism in vulnerable individuals. These findings regarding autism are controversial, however, with many researchers believing that increasing rates in certain areas are a consequence of more accurate screening and diagnostic methods, and are not due to any sort of environmental factor.
Individual substances and toxin classes and their effects
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.
Children exposed to prenatal tobacco smoke may experience a wide range of behavioral, neurological, and physical difficulties. Adverse effects include stillbirth, placental disruption, prematurity, lower mean birth weight, physical birth defects (cleft palate etc.), decrements in lung function, increased risk of infant mortality.
Elemental mercury and methylmercury are two forms of mercury that may pose risks of mercury poisoning in pregnancy. Methylmercury, a worldwide contaminant of seafood and freshwater fish, is known to produce adverse nervous system effects, especially during brain development. Eating fish is the main source of mercury exposure in humans and some fish may contain enough mercury to harm the developing nervous system of an embryo or fetus, sometimes leading to learning disabilities. Mercury is present in many types of fish, but it is mostly found in certain large fish. One well-documented case of widespread mercury ingestion and subsequent fetal development complication took place in the 1950’s in Minimata Bay, Japan. Used by a nearby industrial plant in the manufacture of plastics, methyl mercury was discharged into the waters of Minimata Bay, where it went on to be ingested regularly by many villagers who used the fish living in the bay as a dietary staple. Soon, many of the inhabitants who had been consuming the mercury-laden meat began experiencing negative effects from ingesting the toxin; however, the mercury especially impacted pregnant women and their fetuses, resulting in a high rate of miscarriage. Surviving infants exposed to mercury in-utero had extremely high rates of physical and mental handicaps, as well as physical abnormalities from exposure in the womb during key stages in fetal physical development.  The United States Food and Drug Administration and the Environmental Protection Agency advise pregnant women not to eat swordfish, shark, king mackerel and tilefish and limit consumption of albacore tuna to 6 ounces or less a week.
Mercury exposure in pregnancy may also cause limb defects.
The developing nervous system of the fetus is particularly vulnerable to lead toxicity. Neurological toxicity is observed in children of exposed women as a result of the ability of lead to cross the placental barrier. A special concern for pregnant women is that some of the bone lead accumulation is released into the blood during pregnancy. Several studies have provided evidence that even low maternal exposures to lead produce intellectual and behavioral deficits in children.
Dioxins and dioxin-like compounds persists in the environment for a long time and are widespread, so all people have some amount of dioxins in the body. Intrauterine exposure to dioxins and dioxin-like compounds have been associated with subtle developmental changes on the fetus. Effects on the child later in life include changes in liver function, thyroid hormone levels, white blood cell levels, and decreased performance in tests of learning and intelligence.
Compounds such as carbon monoxide, sulfur dioxide and nitrogen dioxide all have the potential to cause serious damage when inhaled by an expecting mother. Low birth weight, preterm birth, intrauterine growth retardation, and congenital abnormalities have all been found to be associated with fetal exposure to air pollution. 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 toxins. 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.
Control of air pollution can be difficult. For example, in Los Angeles, regulations have been made to control pollution, by putting rules on industrial and vehicle emissions. Improvements have been made to meet these regulations. Despite these improvements, the region still does not meet federal standards for ozone and particulate matter. Approximately 150,000 births occur every year in Los Angeles. Thus, any effects air pollution has on human development in utero are of great concern to those who live in this region.
Pesticides are 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. Overall, the two pesticides most frequently detected in cord blood are diethyltolaumide ( a commonly used repellant) and vinclozolin (a fungicide). 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. 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.
- Heat and noise have also been found to have significant effects on development.
- Carbon dioxide – decreased oxygen delivery to brain, intellectual deficiencies
- Ionizing radiation – miscarriage, low birth weight, physical birth defects, childhood cancers
- 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.
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.
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