Environmental toxicants and fetal development

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.[3]

Toxic substances that are capable of causing structural congenital abnormalities can be termed teratogens.[4] 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.[5] Teratogens are classified in four main categories:

• Drugs in pregnancy – in addition to environmental chemicals, includes recreational drug use and pharmaceutical drugs.
• 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

Neuroplastic effects of pollution can give rise to neurodevelopmental disorders.

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.[6]

Drinking alcohol in pregnancy can result in a range of disorders known as fetal alcohol spectrum disorders. The most severe of these is fetal alcohol syndrome.[8]

Fetal exposure to prenatal tobacco smoke may experience a wide range of behavioral, neurological, and physical difficulties.[9] 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.[7]

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.[10] 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 1950s in Minamata Bay, Japan. Used by a nearby industrial plant in the manufacture of plastics, methyl mercury was discharged into the waters of Minamata 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 intellectual disabilities, as well as physical abnormalities from exposure in the womb during key stages in fetal physical development.[11] 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.[10]

High mercury levels in newborns in Gaza are theorized to originate from war weaponry.[12]

Mercury exposure in pregnancy may also cause limb defects.[3]

Adverse effects of lead exposure in pregnancy include miscarriage, low birth weight, neurological delays, anemia, encephalopathy, paralysis, blindness,[3][7]

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.[13]

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.[14]

Air pollution can negatively affect a pregnancy resulting in higher rates of preterm births, growth restriction, and heart and lung problems in the infant.[15]

Compounds such as carbon monoxide, sulfur dioxide and nitrogen dioxide all have the potential to cause serious damage when inhaled by an expecting mother.[16] Low birth weight, preterm birth, intrauterine growth retardation, and congenital abnormalities have all been found to be associated with fetal exposure to air pollution.[17] 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.[16]

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.[18]

Particulate matter (PM) consist of a mixture of particle pollutants that remain in the air, and vary be region. These particles are very small, ranging from PM10 to PM 2.5, which can easily enter the lungs. Particulate matter has been shown to be associated with acute cardio-respiratory morbidity and mortality. Intrauterine growth has been shown to be affected by particulate matter, leading to unhealthy outcomes for fetal development such as poor or slow fetal growth, and increasing fetal morbidity and mortality.[19] A study from 2012 found that exposures to PM 2.5 differed by race/ethnicity, age, as well as socioeconomic status, leading to certain populations experiencing greater negative health outcomes due to environmental pollution, especially relating to particulate matter.[20]

Pesticides are created for the specific purpose of causing harm (to insects, rodents, and other pests), pesticides have the potential to cause 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 an infant's cord blood, proving that such toxins are indeed transferred into the baby's body.[21] Overall, the two pesticides most frequently detected in cord blood are diethyltoluamide (DEET) and vinclozolin (a fungicide).[21] 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.[21] In 2007 alone, 1.1 billion pounds (500 kt) 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.[21]

A 2013 review of 27 studies on prenatal and early childhood exposures to organophosphate pesticides found all but one showed negative neurodevelopmental outcomes. In the ten studies that assessed prenatal exposure, "cognitive deficits (related to working memory) were found in children at age 7 years, behavioral deficits (related to attention) seen mainly in toddlers, and motor deficits (abnormal reflexes), seen mainly in neonates."[22]

A systematic review of neurodevelopmental effects of prenatal and postnatal organophosphate pesticide exposure was done in 2014. The review found that "Most of the studies evaluating prenatal exposure observed a negative effect on mental development and an increase in attention problems in preschool and school children."[23]

In 2017, a study looked at the possible effects of agricultural pesticides in over 500,000 births in a largely agricultural region of California and compared their findings to birth outcomes in other less agriculturally dominated California areas. Overall, they found that pesticide exposure increased adverse birth outcomes by 5–9%, but only among those mothers exposed to the highest quantities of pesticides.[24]

Benzene exposure in mothers has been linked to fetal brain defects especially neural tube defects. In one study, BTEX (Benzene, toluene, ethylbenzene, xylenes) exposure during the first trimester of pregnancy has been clearly indicating negative association with biparietal brain diameter between 20 and 32 weeks of pregnancy. Women with high exposure to toluene had three to five times the miscarriage rate of those with low exposure, and women with occupational benzene exposure have been shown to have an increased rate of miscarriages. Paternal occupational exposure to toluene and formaldehyde has also been linked to miscarriage in their partners. Normal development is highly controlled by hormones, and disruption by man made chemicals can permanently change the course of development. Ambient ozone has been negatively associated with sperm concentration in men, chemicals associated with UOG operations (e.g., benzene, toluene, formaldehyde, ethylene glycol and ozone) have been associated with negative impacts on semen quality, particularly reduced sperm counts.[25]

A 2011 study found a relationship between Neural Tube Defects and maternal exposure to benzene, a compound associated with natural gas extraction. The study found that mothers living in Texas census tracts with higher ambient benzene levels were more likely to have offspring with neural tube defects, such as spina bifida, than mothers living in areas with lower benzene levels.[26]

• Heat and noise have also been found to have significant effects on development.[7]
• Carbon dioxide – decreased oxygen delivery to brain, intellectual deficiencies[7]
• Ionizing radiation – miscarriage, low birth weight, physical birth defects, childhood cancers[7]
• Environmental exposure to perchlorate in women with hypothyroidism causes a significant risk of low IQ in the child.[27]